Turbos go where the food is, be it glass or rock; mine are my primary cleaners of both. If there is more/better food on the rock, they will go there first. This is true of most snails. Might try a couple more of them and see if it spreads them out a bit more due to depleting the rocks a bit. Trochids are also good, but they seem to be less commonly available. If you've got a massive algae attack on your glass, particularly if its encrusting, Ceriths might crawl up and have a munch, but won't make a dent overall due to their small size and itty bitty mouths. You'd have to have a not-so-small army of them to do much successful glass work.
Also, if it's encrusing algae you're dealing with, elbow grease with a scraper is another important CUC member, since even very large snails struggle to get rid of all of it from an area.