If this is your only rock with algae, it probably came infected with it. Take it out of the tank, and saturate the algae with hydrogen peroxide. Leave it out of the water for about 5 minutes. After about 3 minutes, saturate the algae with hydrogen peroxide again and let it sit for the last 2 minutes.
After 5 minutes is up, place the rock back in the tank. There is a large thread in the Nano Tanks section called pest algae removal challange with tons of positive results.
After about 3-4 days, the algae will be about 80-100% gone. Anything that survives, just repeat the process on. This can all be gone in less than 2 weeks, with about 10 minutes of work.
If it comes back...then you may have a water quality issue to address...but this will be a quick fix. DO NOT get the hydrogen peroxide on the coral or base, and remove any snails, hermits, or creatures that may be on the rock also. I have done this on everything from infested sps frags, to large 10lb rocks covered in hair algae.