From what I've seen in other folks' reefs, "Acans" (now officially Micromussa lordhowensis) will spread over their plugs, and if they find other rock beneath, they'll just keep spreading, encrusting the surrounding rock. If they find sand beneath the plug, though, they'll tend to mound up, forming something of a ball shaped colony.
HTH
~Bruce, whose "Acans" are still just little . . .