I need an ID on this Odd thing

[Peaches0013]

https://www.dropbox.com/s/j6j8ndirdd..._4900.JPG?dl=0

It moves like a scallop, but I don't own any, I had a nano running for almost a year transferred my rocks over to my 120. I also purchased new rocks(dry) for my 120...
However,
the other night I spotted this "thing"
Looks like a scallop, moves like one, but has NO shell..
Loves to hide in the dark, doesn't fancy the light at all... It seems to filter feed..

I have been searching forms I can't anything, I haven't seen anything like it.

I want to know if its good or bad.