Tony,
Your picture shows a Maxima clam (less likely Squamosa) that is in very poor shape. I don't think he is going to make it. He looks like he had recent growth.
I don't think that a Ca level of 550 cause this problem.
You did not give me much information so I cannot tell you very much other than the above. Regardless, I don't think your clam will make it. Often, by the time the clam looks bad, there is nothing you can do for him. I am sorry.
You mention recent flatworm out break but I see no flatworm in the picture. How did you get rid of the flatworm?
How long did you have the clam? How big is your tank and how do you light it? What is the rest of the water parameters?
Do you have any other clams in your tank? Is there any fish that bother the clam?
If you give all of these information’s, maybe I can try to tell you what may be wrong so that you have less chance of loosing another clam.
Sorry,
Minh