On Dwarf angels: They will pick at everything, and a lot of people take this as them "nipping" or attacking corals. They are substrate feeders, and will eat algae,pods,etc off of anything. The problem is, they've gotten a very bad reputation, and anytime anyone sees them near a coral, its "OH MY GOD!!!11!!!one, Hes NiPPING my CORALS!!"
They eat in a manner very similar to a mandarin. Although if someone said a mandarin was "nipping" thier corals, they'd be called daft. Its all reputation. They just kind of wander around and take a swipe at everything. For about 80% of the angels I've seen, it never gets any worse than this, and it doesnt bother the corals.
Some of them do get bad, but probably 80% of the ones that get called bad are incidences of "OMG, IT TOUCHED MY CORAL AND ITS AN ANGEL!11!!!1one." A bad reputation can be a scary thing.
As for reputations, I've never heard anyone say Tangs are bad with clams, but I think every LFS I've been to that has a tang in its clam vatt, I've seen the tang attacking the clams. Again, they dont have the reputation, so they dont get called on it.
Again though, there is a chance that a dwarf will start becoming a problem, just dont freak out the first time you see him pick at a coral, wait till you see damage.