def. Nudibranch. as earlier people posted, impossible to tell you good/bad without clearer picture.
Now; Nudibranches come in thousands of varieties, each of which pretty much has their own diet.
- There are Berghia Nudibranches, they eat aiptasia, they're the good guys!
- There are Zoanthid eating Nudibranches. Bad guys
- There are Acro eating Nudibranches. Bad guys
- etc..
I would assume bad guy, to be safe.. and if you have had issues with Zoanthids lately, here's your culprit.
-Rudy