First of all, a happy clam is always good to hear about.
Coral growth on clam shells does not appear to be detrimental in any way to clam health. In fact, this occurs in nature all the time. However, there are a few things to consider. A lot of LPS corals have particularly vicious nematocysts to fend off other corals. These may or may not sting your clam's mantle. Some do, some don't. And, croceas in the wild typically have nothing growing on their shells since they would be almost completely lodged in coral or rock at that size.
If your coral inhibits light to the clam mantle, it obviously must go. Watch the edges of the mantle that touch the coral - has the flesh in these locations retracted any or show signs of damage? If it has, the coral must go.
You can accomplish this many different ways. You could remove your clam from the water (On his side!), and scrub the coral with a toothbrush or similar tool - then rinse away what you scrub. Yes, a good shell scrub/massage will kill the coral.
But, if your clam is happy, don't bother him. Just keep an eye out.