you can try to pry it out of there..some will release easier than others.but if it's attached on 3 sides..it probably will break.
i had 3 emeralds..only 1 left.. i have never actually seen them feed on the valonia,, just continue to pick at things within rock crevices..it's like they don't even see them all over the rocks.
but,,i have noticed a few empty white skinned algae bubbles..so who knows.