He's almost certain they should be dead but his knowledge of barnacles, while it might be a little greater than Gorgug's, is still extremely limited. Maybe they weren't aquatic after all!

The barnacles are stared at silently for a long time. WAS it safe? Would that be introducing an invasive species to the waterways? Wasn't that a bad thing? Or was that mussels.

This is the kind of thing Sephiroth is pretty sure should be researched before being acted upon. "I don't see why not," he concludes, reasonably sure it had to be mussels. What could go wrong, introducing perhaps indestructabel barnacles to Etraya? "You don't have to really worry much about putting a fish in water, so it should be the same principle once they're ... actually off.."