could be a couple of things spunges.

May even be a very small rock Nem that comes out at night they have some color to them. check at night with a flashlight to see, if it is one of the little rock Nems they are harmless and only come out at night they do not like light. They syay small I have a coulpe in my tank right now one has purple or blue huw to it and the other has a bit of brown tint to it. These two are now atapsia or mojano I've poste pics before and have confemed this. Besides atapsia and Mojano will come out during the day in bright light these other one do not like light.

A better pic would help?

Bill