Green spiny critter

[Laluna579]

I’m new to reefs and noticed this green spiny critter wander around my live rock. At first I thought he was a zoa because he hung out on top of there for a while and he’s the same exact color, but now he’s migrating. Any thoughts would be great.

 

[kchristensen8064]

Looks like a nudibranch of some sort. I don't know much about them so I can't tell you if it's a good one or not, but to be safe it might be wise to catch it and isolate it until you know for sure.