this is a red kelp, it's a fast growing macro algae, very beneficial for your tank but it should be kept in a refujium to control it's growth, it probably came on the life rock, you should never wash life rocks with fresh water, you killed all the good bacteria that been living on your rocks for months or years, and the algae will come back, you should remove it manually before it spreads and cover the whole tank or get a tang, tangs eat them