do you need to use it as live rock or would it be safe to kill the rock to kill the algae

if you don't mind it going into the tank as dead rock then I would say boil it as it will kill any algae on the rock

other wise scrape it clean as you can but it is very possible that the algae will bloom again

or fill the bucket with sw and get the mg up to about 2k in the bucket just in case it is bryopsys or some other demon grass