They'll eventually grow over it, yes. They grow over any solid surface you offer them. They'll encrust onto rock if you let them.
If you want to take them off the plug, though, it's not too hard. You take the plug out of the water so the acan shrinks up, give it time to retract all its flesh, and then use something like a pair of wire cutters to snip right at where the acan skeleton meets the frag plug. The glue used to attach them will pop right off.