dwarf angels are really quite reefsafe, especially flame angel. none of the dwarf angels eat corals in the wild and certainly don't eat tridacna clams. In our tanks, they may sample corals on occasion but it's usually nothing serious as this is not their main diet. however some species tend to peck more and eventually the coral will die not because it's being eaten alive, but because of stress and damage. one good example is the lemonpeel angel which loves pecking on LPS.
thus the occasional pecking will not do anyharm to corals and clams. however, if the pecking is very often on your clam, your clam will close up and eventually die.
so you be the judge. isit doing it so often that the clam is very stressed and does not want to open? if so, remove the flame angel or remove the clam.