In my experience pom poms have been quite reef safe. Maybe one of the only indisputable reef safe crabs? Possibly?
I have had 4 of them in my mixed reef for a while now (although I only see them at night), they are quite fun to watch. They are way too busy hanging onto their pom poms (actually tiny anemones) with dear life to bother anything with their pinchers
Every time i see them I always sing in my head.. "hey mickey you're so fine, you're so fine you blow my mind hey mickey!" Right.. anyway, carry on.