Heavenly Damsel is right about one snails/ crab per 1-2 gallons. But since your algae problem is on the glass, the crabs won't be able to help you much. Get the snails, they're helpful and fun to watch. Trochus is good, because they can flip themselves upside down when they're turned upside down (falling from the glass). Other snails cannot, and you have to help flip them over when that happened, to keep them alive. I have astrea snails.
Another option, get that magnet cleaner to clean the glass
. How long do you keep your lights on anyway? Any excess nutrient? Because those might cause you algae.
stupid_naso