Is the clam still doing well? I don't see anything in that fish list that is throwing up any red flags other than maybe a tang trying to get at some algae on it's shell.
I am not sure the dip or feeding would help. I don't think clams are able to consume large particles like reef chili. I would make sure nothing is harassing it and then leave it be. Could it be acclimating to higher light?
After 11 years I've gotten ok at it. At some point you learn if you keep quality parameters the vast majority of organisms will thrive even if there is something that goes wrong temporarily. I've also learned that alot of the time things can die for no **** reason!
Spot feeding or FWD is just going to make matters worse IMHO. Clams don't need fed. Are you running carbon in the tank? If not I suggest adding some and run it hard and change every few days for a week. Can you post an update pic?