first i would make sure ur water is all in line. then make sure that there isnt anything bothering this clam, like a fish(angels),shrimp,pyrams or anthing else. that pic is kind of typical of PM but the rest of the mantle looks fine. most of the time u will see some curling on the ends of the mantle before the clam pulls it mantle in at the incurent opening.
for a FWD u need to use RO/DI,=temp and PH to tank and place clam in there for 20 to 30 min.check everything else first before u dip