Off-colored T. Derasa

[flamboyantcuttle]

Hi all, we have a 4.5k gal tank with two Tridacna derasas, both about 5 years old and from the same farm. Both clams are in the same tank with a single yellow tang and a collection of shallow water corals. I've attached pictures so you can see how healthy one is, and how unwell the other looks. He still opens to filter feed and reacts to touch, but has barely been growing. It seems to almost have a "slipped disc"? We put it in a Petri dish to try and sit it up, but it keeps slumping and going "cross eyed" in terms of the shells not meeting symmetrically. We would appreciate some advice if anyone possibly knows what's happening or ways to replenish its color or sit it up straight. We're confused, as the other derasa is doing so well in the same tank!
Thank you so much

The ill clam (we're doing frequent aiptasia and algae removal)

The healthy clam! (Picture does not do its brilliant color justice)

 

[Shirak]

Sounds like the one might have a broken hinge. I would examine and if so you can find some threads here on broken hinges.

 

[Outlaw Corals]

look’s like a issue with it’s bissell glad, it look’s very agitated which would explain not staying upright, were the clams attached to the glass, and what is that thing or whatever it is on the Clam ?