This often occurs on soils that stay damp as a result of an overly peaty (peat-moss) mix.
Long-term, you'll want to re-pot the plant into a gritty, free-draining mix that won't support molds or fungus, et cetera.

Short term, you could try adding a touch of vinegar, lemon juice, unscented dish soap (or even an actual soap-product intended for plants) to your water. I would start with a capful of vinegar in a gallon of water. Moisten with plain water, then apply the water/vinegar mix to the affected areas. I think this is mild enough that it won't harm your plant, but you could do a follow-up rinse a
few minutes after application.

Josh

ok, thanks, i have just tried the water with vinegar mix so will see how it looks in a few days and then re-apply if necessary. if i re pot the tree what soil mix is best, i have read a lot of stuff online and all seems very confusing and sometimes conflicting. you said a gritty mix, what do you mean by this exactly? and ive read that peat holds too much water, should i have no peat in the soil at all?? its going to be kept indoors all year round, i live in the uk (just to give an idea of the amount of sunlight it'll get). would covering the top surface of the soil in stones help (preventing the mould getting sunlight)? and can you get the stuff needed for the soil mix from regualr garden centres or do u need to buy from a specialised retailer? i know this is a lot of questions but as i said im a beginner and i dont want my poor tree to suffer as a result!!

You need to get more information on indoor bonsai. Go on line and look for books on the subject. A couple of authors to look for would be Jerry Meislik and Paul Lesniewicz. If you are really serious about doing bonsai you should join a club where more experienced people can help you one on one. You can find clubs on the internet rather easily. I'm sure that most of the UK clubs have web sites and can be located with a Google search or some such. As for your soil mix, it almost certainly has too much oganic stuff in it which is decaying, hence the mould. Gritty mixes contain about 80-90% coarse sand or crushed lava with a particle size between 2-5 mm. This is needed for good drainage. I don't know what is available in the UK because I live in the US but I'm sure you can find something suitable. You could probably do a repot at this season if you removed about half of the foliage at the same time, by pruning. Good luck, Jack