I have them and have extracted one. I hae a few more in the tank. I couldn't find any info on them good or bad. I need to know if i can leave them or if i should take them out.
Naw, they are harmless. I have them all over my tank, ever since I started my refugium they proliferated. They primarily eat detritus and small particles that they grab with their tentacles. Here is one of mine in my fuge --