Serissa top the list as the most difficult of all indoor Bonsai to care for, especially for someone with not much or no experience in tropicals. One of the things this plant is suffering from is stress, it's just had a major change in its environment.
The leaf discoloration suggests a build-up of salts in the soil from 'underwatering'. Watering should be increased to the point that it runs out the drain hole(s) in the bottom of the pot then everything allowed to dry until the soil is just mildly moist before it's watered again.

They don't like being in a pot which is too small, don't like to be re-potted and don't like to have their roots pruned.
Serissa thrive in humidity of about 65% or higher, the higher the better. People who sell humidity trays don't tell you that, in the average home in a temperate area, yours would have to be the size of a bathtub to do much good.
If you should decide to re-pot it I will suggest that you do this without disturbing the roots (or as little as possible). It can be 'slip-potted' into something like a larger bulb pan or a shallow nursery container. They will grow very well in a normal but slightly coarse potting soil which can be found at any garden center.

I will also suggest that you Google Serissa care, there is a wealth of information on the web. Good luck.

Bob

Serissa is one of those plants like Carmona microphylla (fukien/fujian tea). People either can't grow it or they have no trouble with it. I hate Carmona because I can't keep the scale away from it, but I have several Serissas that remain healthy under fluorescents during the winter & in full sun from last to first frost. I start dozens of cuttings (they come soo easy from cuttings) for others, and they all do well with them. I'm sure I have at least a dozen friends with Serissas that I regularly shear into something that looks a little like bonsai (but looks great to them) for them a couple of times each year.

Anyway, before I get too far off the track ...... It's difficult to guess at what might be ailing your plant. I suspect a reduction in light is the culprit (ask if you want a technical explanation), but over-watering, under-watering, and a high level of soluble salts in the soil resultant of the practice of watering in sips - which ensures the soluble salts in tap water & fertilizer remains in the soil - all bring on the exactly same symptoms because they all cause the same drought response in trees. Burned leaf tips and margins are usually by-products of over-watering or excessive salt levels in the soil in plants like Serissa with medium-thick cuticles (leaf skin).

I have trees in training pots and small bonsai pots, there doesn't seem to be any difference in vitality between the two, though the plants in larger training pots put on mass much faster than those in bonsai pots. Soil is the key if your plant is in a small pot. The shallower the pot, the more essential fast drainage is. If you're to become truly proficient at bonsai, somewhere along the line you'll need to have a firm grasp of soil/plant relations. If you're interested, I can provide a link that has information you can apply not only to bonsai, but to all your container growing endeavors.

It's not unusual for Serissa to throw all its leaves at you in a huff, then pout, so there is plenty of hope. Here is what you might do to get your tree back on track:

I would repot asap, which means this spring. Learn about soils before then and gather your ingredients to make a suitable soil - I can help with that if you're interested. In the interim, I would move the tree outdoors. You don't say where you live (please consider adding something like I have next to my forum name at the top of this post), but Serissa doesn't mind cool temps down to frost levels, but avoid frost. When you water, water copiously so at least 15%+ of the water applied exits the drain hole(s). If your tree can go more than 3 days between waterings, you need to do something to facilitate drainage. Inserting a wick into a drain hole and tilting the pot at a 45* angle after watering should solve the issue of an overly water-retentive soil. Fertilize often (weekly or every time you water - ask about reduced dosages and some direction before you embark on this program - I'll explain) while the tree is actively growing. I suggest a fertilizer in a 3:1:2 NPK RATIO. Ratios are different than NPK %s. 24-8-16, 12-4-8, and 9-3-6 are all commonly found 3:1:2 ratio fertilizers. This ratio, which is selected by the manufacturer to come closest to to supplying nutrients in the same ratio plants use them, is probably the very best choice for developing plants.

Finally, if you need to treat the plant for insects or disease, and you're unsure of the exact diagnosis so you can target the malady specifically, try using Bayer's 3-in-1 product. It contains Imidacloprid (a systemic insecticide), Tebuconazole (a systemic fungicide), and a miticide (Tau-fluvalinate? [sp?] or something like that). It is an excellent product for the shotgun approach, and since I've used it many times as a prophylactic and fixative on my Serissas, I know they don't react negatively to it. If you use it - spray late in the day when the next day promises cloudy conditions or protect from sun for a couple of days subsequent.

http://www.bayeradvanced.com/tree-shrub-care/products/3-in-1-insect-disease-mite-control

Good luck. Questions?

Al