Yes some blennies--midas blennies in particular are known to nip at clam mantles. Correction, not midas blennies, I meant Bi-color blennies. Midas are fine IME.
This is the first place I am hearing about melanarus wrasses eating clams. I've never seen or heard of it anywhere before so I'd have to assume that the clam was either on it's way out or the wrasse had some sort of dietary deficiency it was trying to correct. The same way our brains make us crave foods with substances we need when our levels drop, fish go after coral tissue on occasion to fill a gap in their diets.
In my experience the missing food item is greens. Even with carnivorous fish, they need a vegetable aspect in their diet since they get this in the wild from the animals they consume feeding on algae. Since I doubt you have a tank full of algae eating flatworms for the melanarus to feed on you need to supplement with algae in it's food.
A lot does not translate to "well". If you can get your hands on some type of phyto that you can add to your frozen food (I like phytofeast since it's liquid and easy to work with) you will see a definite difference in your fish color and health wise.