I have two natural ponds, one is fed from surface water run off, the other is fed from a small creek in a swamp. Both water sources are fairly nutrient rich so there's always some algae. Where most of it grows is in shallow sunlit areas. Deep or shaded areas have minimal algae. There are shrimp that feed on algae, but I don't know whether there are any suitable for your climate. Waterlilies covering the water surface will block sunlight and reduce the algae. So instead of seeing green masses of algae you'll see masses of green waterlily leaves, far preferable to most people. I've also found Lotus (Nelumbo species) reduces algae and still leaves the water visible for you. The frogs wouldn't mind either way though. Some tadpoles like to eat algae, and some will eat waterlily. Nature copes in its own way.