That doesn't look like PM to me ... when I've seen PM, it's been pinched back all the way to the shell. This does seem more like it's curling rather than pinching.
However, having said that, it does seem suspicious if you've had PM recently in another clam. Clams do share communicable diseases quite easily - if you lose one clam in a tank it's a sure bet to lose another in short order.
Sooo .. weighing the options, I would do the FW dip. Basically it comes down to this: the FW bath won't hurt the clam long-term; but if it IS PM, that will kill the clam eventually..
Some good info about PM here:
http://reefkeeping.com/issues/2006-09/jf/index.php