Strange spot on one of my walls. Is it mold, and what can I do about it if it is?

The spot looks like dust. Thats exactly what I thought it was so I just swept it off with my broom. I did that like a month ago and now, it's back. The reason I suspect its mold is because my wall is moist. I know that where there's moisture, mold likes to grow. I saw something on the news once about mold and how it can kill you (I think it was black mold) and now I am terrified of mold. Anyway, it stops right where the trim between the floor and the wall. Also, there's this spot on my floor (hardwood) that feels really cold and is darker that the rest of the wood around it. Its in the same area as the spot on the wall just on the floor. Is there a specialist I can call to check this out or can I do it myself (which I don't wanna do because I'm scared of it) also if this is mold I'd have to fix the problem but, will it be expensive???!!!!!