Any living creature or plant I've tried to place in one of my turtle aquariums, they've eaten. Even those little snails that produce at 5x the rate as bunnies, become extinct with my turtles. Good for the turtles, but not for the algae. Algae, like plants, needs 3 things to survive; water, light, nutrients (from soil, or in this case-waste).
If you're having algae problems, odds are, your phosphates are high (due to waste), and the area it is thickest gets the most light or if it's evenly distributed, your water is (too) warm.
Turtles, particularly Sliders, are easily adaptable to environmental changes--when not too drastic.
I suggest (if possible): Raising your lighting, adjusting light time/frequency, removing/lowering the temp of any heater, partial water changes daily (even if only a gallon) WITHOUT adding dechlorinator, vacuuming out any solid waste/dirt sitting in corners or lying on the bottom of the tank.