ID this hitchhiker

[Nano Gabe]

It’s the second one I’ve found it has a hard body and curls up a little once messed with.

 

[Reefer Reboot]

Chiton, good CUC.

 

[Nano Gabe]

Chiton, good CUC.

Are they bad?

 

[Reefer Reboot]

Most Chitons are algae grazers. There are a very few that are predatory. One from the western Pacific is, but it has a pinkish mantel. To me, yours looks okay. Maybe someone else knows the exact species.