Does the other clam also have PM? Bummer.
I'm not sure what I would do. Is there anything attached to the rock (other than the clam that is) that you don't want to lose? I suppose that would do a number on the coraline ... but it will come back eventually.
What about chiseling part of the rock off? I know it sucks, but ... the clam is worth it, right?
I don't know anything about cutting byssal glands. I suppose the clams we buy at the stores have been cut off something, and they eventually regrow, but ...... I dunno, the thought about cutting the clam off the rock gives me the heebie jeebies (I wouldn't do it if it were me)..