A pic of the algae and snail would be great. You will most likely need more snails to keep the algae under control. Green film algae on the glass is quite normal but too much too fast is also a sign of higher than desirable nutrient in the system.
The blenny will definately help with this problem some but the snail will be the primary consumer. You should consider some cerith as well for the sandbed.
Bi colors will often stake out a territory where the "back into" a hole or crevace in the rock. If the damsel is getting to close, the blenny willl give chase but the damsel will soon learn to steer clear.