Derasa clam growing a notch in its shell.?

[LazyFish]

So had this guy a bout a month and about 3 days after I got it I think something may have stung its mantle. It dosnt look like pinched or rolled mantle disease to me. That was actualy my knee jerk reaction as to what it was but this is my first clam so idk what it realy is. It isn't spreading or progressing at all over the past weeks I've had it and is very localized. It's been like this since then with no signs that it's getting worst it actualy looks a bit better than it first did. The efected area can extend and contract with the rest of the mantle just dosnt go as far out as it should almost like a scar tissue stricture or something tugging at the flesh. About 75%, of it's new growth has appeared while in my tank. You can see the spot on the shell where the injured place is at is not growing as fast as the surrounding shell and making a small notch. It seems otherwise healthy. Any one seen this before or think it could still be

Pinched mantle?

First pic showing the notch second how the mantile looks fully extended and the last shows how well localized the area realy is.

[EasyEd77]

It has been a long time since I was in the hobby but I am thinking of getting back in. To me it does look like pinched mantle.  When my clams did this I would do a freshwater ro/di dip. Google treating clams like this.  I remember it worked at getting the clam healthy again and was very easy to do. Even if it is not pinched mantle it will not hurt the clam so you might as well try. 

[LazyFish]

It actualy seems to be resolving itself. I've been keeping a close eye on it and the notch is filling in and the crease is almost gone now. Im leaning towards something may have stung it.