White powder on bedroom wall (Efflorescence?) and black ring??

So since about February-March, there's been a slight smell in my bedroom. I didn't really recognize until a few weeks ago, which has since then gotten stronger and you can clearly smell it when you enter the room. There is some white powdery stuff on my interior walls which rubs off easily but isn't fluffy (like efflorescence) yet I can't think of what else it can be. I admit I rarely have my windows open (because whenever I do there's like 3 more spiders and a few other insects added to the household :/) and I keep my door closed most of the time. It is only in two corners of my room, one of which has a black ring (or atleast a dark ring) with a lot of the white stuff in the same area and a few small black dots (which also surround my window). Basically, I think these are all related to damp/moisture in the room. To help the smell, I regularly open my window (I deal with the insects in my room now) and leave my door open more often. What is the dark ring, and how do I get rid of the problems? I'd like to try and solve them with homemade/cheap/easy ways first before the professionals come in haha :) Thanks