It's a little hard to see the duster in your photo but if it has a mucus tube it will probably get bigger and then rehome. If it has a calcified tube make sure it is really a duster. Very small dusters are common in/on live rock and they stay small. It will probably just stay on the snail. The reason I said to check is to be sure it is really a duster type crown. It could be a vermetid snail which doesn't really have a crown it is more like strings. If it is a vermetid, pull the snail out and scrape it from the shell. They can reach nuisance levels.