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
I would assume bad guy, to be safe.. and if you have had issues with Zoanthids lately, here's your culprit.
Are you talking about that bright white thing? On the post is says that’s just a white thing on the glass? Because I think that’s what everyone is referring to as a nudibranch. Just trying to make sure.