damsels are very teritorial and will defend their teritory at all costs. if there are no rock formations they can claim as their own they will dig out a home under anything they can, in your case the heater.
not the best tank mate to have, they will have a go at anything that wanders into their bit of reef regardless of their size. keep an eye on your other stock, if you see their fins getting niped please take the damsel back, it will more than likely kill something