The pic is so small its hard to make out any detail. If it is in fact bubble algae. Then the best means of control in a Nano is manual removal. Remove the rock if possible carefully pick off the bubbles, try not to pop any, this can be tricky. Give the rock a good rinse in some saltwater, andplace it back in the tank, then watch for any more bubbles. If the rock cant be removed then your gonna need to remove the buubles under water, I use a siphon tube near where I'm removing the bubbles to remove any spores that might get released when a bubble gets popped.