[Delphinus]

Wellll .... don't feed them to the mantis. Someone will take them off your hands if you don't want them.

It seems to me if the corals were being damaged, you'd know it, ie., you'd see the damage. The slime, in and of itself, isn't really indicative of damage. My guess (it is really only a guess) is that the corals aren't used to the sensation of the little prickly legs, and as such are reacting. But they should acclimate eventually: many acroporas in the wild house commensal crabs, these crabs never leave the coral and the coral is fine. So you'd "think" they'd be able to get used to such a thing.

Just my $0.02...