No one can tell you what the primary cause is without a lot more information. It's almost certainly a combination of poor soil, inappropriate watering practices. The spots are scale insects.

Hi & welcome to Gardenweb! From what's visible in the pic, your Scheff didn't get that way in just a month, but too much water could have gotten it to the point where it's much more noticeably unhappy. If you don't have anywhere with more light for it, improvement would be iffy, if possible at all.

"First, I have a black thumb and have tried very hard to keep a few plants alive and healthy on my patio, which gets no direct sun and only has light shade." There's no such thing as a black thumb, just an uninformed grower. Also, if the conditions aren't suitable to a plant, it won't do well, regardless of what thumbs are around. Many plants do well in lower light. Having some that like it, instead of struggle, would be a lot rewarding for you, IMHO!

Had a larger sheffy last year that totally succumbed to scale after everything I tried. Sheffys are SO susceptible to them and once you have them OMG it's so hard to get rid of 'em.

The remaining plants that had scale went outside when it warmed up and the natural method kicked into effect(their colonies were discovered and devoured by predators).

Fight the good fight.
It's worth trying(although I certainly wouldn't hold it against you if you tossed it and got a replacement).

Good luck! :)

Thank you all. This plant was suggested to us by a nursery as doing well in low light. We've been trying different plants for three years now, built boxes for them rather than planting them into the ground, followed fertilizing instructions and yet still have had little success. Our elephant ear and clivia are doing well. As to the schefflera, I've been given so many different opinions on watering from different gardeners since then that I am sure watering practices are part of the cause. Despite this, I will try to fight the scale as I am sure my husband will put his foot down at any more plants if I have to toss this one! So sad as it was so perfect for the space the past couple years, it masked an ugly wall.

On second thought, after reading more about scale, I think I have to toss it to save what few plants we have that are happy.

Quarantine it away from the others,yes.

Give up? ...No. :)

That's understandable. If you're not a plant-o-phile, it can take a few tries to find the right thing, or at least eliminate what won't work. To hide a wall in low light, nothing fancy or expensive is necessary, you could use heart-leaf Philo trained on the support of your choice, or dangling from a hanging pot.

Hawaiian Ti plant is another good candidate (Cordyline fruticosa) with pretty red leaves. (The tall one in this pic, which is a pretty small plant, compared to some specimens.) Having 3-4 in a pot makes a nice, full shrubby looking thing, (and you'd know better than to sunburn all of the leaves on yours when you brought it home.)

Thank you all!