there are literaly too many things that it could be anywhere from nudibranches to types of bristleworms.
what do you have in your tank as far as inhabitents?
and how are your levels?
I have several types of shrimp in my tank pepermint, camel back, and a cleaner shrimp. As far as my parameters SG 1.025 Nitrate 0 Nitrite 0 Phos 0 Amm 0. Any ideas.