Looks to be a Squamosa but not sure? How long have you had it? Has anything changed in the tank recently? How's the calcium level? They do like a tad bit of nitrates but not overdone. Check the foot area and make sure the shells are not separating or being infested with parasites. Just few ideas.
Flow is not to high, no shell separation, no nasty pyramid snails.
Fish 2 O. clowns
1 saddle back clown
1 six line
1coris
1 yellow tail
1 orange shoulder tang
Ca=440
NO3=0
API tests today
I had a case that my clowns try to host in it or it was just trying to piss off the clam by splashing sand onto it. In which case moving it to a different location will solve that problem. Derasa aren't to hard to care for. Mine survived a tank crash and crappy lighting for 6 monthes, in fact I think it even grew.
I'd be of no use since I'm no clam expert, but I could at least double check your test numbers with different kits. Where in Dallas are you? I'm available tomorrow during the day (though I know that isn't likely to be good for you).