After looking closer at the pic PM disease normally shows the lip rolled toward the center. Yours is not!
Some info that may help you. If you really think its PM then try this
Over the years, reef enthusiasts have learned the common misconception that the only way to treat PMD is with a twenty min fresh water dip. This is not true and more often than not you will cause more harm than good. A fresh water dip is extremely stressful and should only be done as a last resort.
My first step to treating PM is try and see what is triggering it and then remove the trigger. IE: if you have GFO, a nipping fish, stinging coral, etc remove it ASAP. Once that has been done I highly recommend running as much carbon as you are able to and push as much flow through it as you can. Once carbon is running, I change mine out every two days during the first week of treatment. After that, I change it every four days till the PM is no longer present. If running carbon doesn't make a change in appearance of the clam in one weeks time then I recommend doing a twenty min fresh water dip.
When I do a fresh water dip, all I do is take a bowl full of new RO/DI water, float it in the tank for ten to twenty minutes to equalize the temperature. Once the temperature is the same as the display tank's water, I place the clam fully submerged in the bowl of fresh water. Over the next fifteen to twenty minutes, I gently use a turkey baster and blow the water all over the clam to ensure a good washing of fresh water. Once done, I place the clam back into the display tank in the lowest stress location I can find.
I hope you have found this article helpful and if you have any further questions, please post them up and I and others will help where we can.