it will not absorb over time.
it is possible the clam may release of its own accord, or it might not.
if you really, really want to remove it from the piece there's a couple things you can try. AboutAcans already noted cutting (the threads, not the foot, that's an important distinction that bears repeating), but there are some other less invasive ways to detach them, especially if you're able to move/remove them from the water.
one that i used to exceptionally great effect was suggested by moliken way back was taking some saltwater, freezing it, and then crushing it up. remove the clam and the thing it is attached to from the water, and gently pack the ice around the foot area. this should irritate the clam in to releasing fairly quickly.
another way is to place it and whatever it is attached to in the path of a powerhead. not enough to flow hurt it, you don't want it getting blasted, but just enough to encourage it in to thinking that this might not be the best spot for it. which should again coerce it in to gently letting go.
of course, if it's a problem you can always just leave it attached. it won't do any harm other than maybe looking a bit unsightly.
