If it is just there, remove the frag to a bowl of water. Scrape the bubble algae off and then carefully drip a little hydrogen peroxide on the spot. Let it sit for a minute or so, rinse it in a little tank water, and return the frag to your tank.
How long has the frag been in the tank? If the bubble algae is just appearing on a frag that's been there a while, you may already have it in your system and just haven't seen it yet.