If it were me, I'd give it until the weekend before getting concerned. But if there was an aiptasia anemone underneath that spot, that's more or less a slam dunk diagnosis IMHO.
The squamy you picked up for me, looked "disgruntled" for several days after introduction to the tank. Well, actually, come to think of it, he looked terrible! I forgot, he spawned all day the next day. He was definitely spent after that effort. The mantle was shriveled looking for several days thereafter. But he looks terrific now!
As Brad mentioned, if it *is* PM, do the fresh water dip. It works like a charm! Just make sure the temperature is matched. Don't worry about the clam, they can handle the fresh water so long as there isn't temperature shock. There's a story from a few years ago about a guy who did a FW dip on a clam for PM, but forgot about the clam so it was in FW all night - he put it in the tank the next morning and it was fine. I guess they just clamp up tight and ride it out.
But I think the culprit here is the aiptasia.