What do you think this is ?

[nashorn]

Found one of my power heads clog and this was in it.
None of my fish are missing.

 

[FartyParty]

Perhaps a sea slug or nudibranch.

 

[nashorn]

Oh no not my sea hair!!

 

[FartyParty]

If your sea hare recently went missing, i would venture to say you found it. I had a sea hare in my first salt water endeavor that went down the overflow, never again to be seen alive.