I just bought 20 or so dwarf blue legged/ zebra legged hermit crabs. I see two on a zoa colony. It looks like they are cleaning in between the colony, but I'm uncertain. Does anyone know if they are sitting ontop of colony they are just eating what between zoa or the actual zoa?

Most likely they are just cleaning them. just watch for damaged polyps.

I never had issue with blue legs and Zoas, but I have had problems with them pulling polyps out of acros and birdsnests.

Edit: And yes, I watched it happen.

Never had a problem with them eating anything alive.

Everyone has different experiences. More than once would I come home to this big chunky blue leg sitting on top of an acro colony going to town. The first few times I dismissed it, but when the coral began to decline, I pulled him, and all other blue leg hermits out and the acro sprung right back. I only use Scarlett and red leg hermits now. I have a few friends that reported similar issues with blue legs and acros.