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You could try using a chisel and scrape them off the rock, taking a tiny bit of rock with it so you have something to help attach it with elsewhere. Or, you could use a very sharp blade and cut them as close to the rock as possible. Then you can re-attach them elsewhere in the tank, or sell them, etc.
OK, I am going to use a sharp blade to cut them off. It may not be simple because of the hidden spots. I really don't want them, they are everywhere and growing like weeds, even in my sump, it is full of it....