Rut roh, might be the dreaded Valonia commonly known as bubble algae... Does it look like this?
Courtesy of melevsreef.com
If so, there are only two things I know of that eat it... Emerald Crabs and Naso Tangs and with both, your mileage may vary. As your tank is still young, you could remove the rock, remove the algae (taking care not to break it cause you will release spores if you do). Once its removed, rinse the rock in a bucket of saltwater a couple times and put it back in your tank. Luckily its only one bubble so you should be able to stop it if you act now
Courtesy of melevsreef.com
If so, there are only two things I know of that eat it... Emerald Crabs and Naso Tangs and with both, your mileage may vary. As your tank is still young, you could remove the rock, remove the algae (taking care not to break it cause you will release spores if you do). Once its removed, rinse the rock in a bucket of saltwater a couple times and put it back in your tank. Luckily its only one bubble so you should be able to stop it if you act now