After manual removal, if you carefully pour some H202 (hydrogen peroxide) on the affected area it will take care of the remaining bits that have grown into the rock. Give it a quick rinse in a bowl of tank water after about 15-20 seconds and return it to the tank.
Thanks, I'll try that!
Is this a new setup? generally speaking, hair algae growth is a sign of higher than normal nutrient levels, specifically Nitrates and Phosphates. If it is on just that one rock, it could very well be that the offending rock is leaching PO4 and therefore giving the HA a place to grow.