Brownish dust like substance on my coral

[Editour2]

I have a few sps frags in my DT that I have to brush off what appears to be a brown like powder or dust everyday, (I use a tooth brush). This dust/powder seems to smother the polyps. It's not on all of my frags or on any of my colonies. I suspect that it may be dino's but I'm not sure. It's hard to take a picture of it since I don't have a camera that has a macro lens. Any speculation of what it could be and how to eradicate it?