Try moving the tank away from the window. The sunlight might be encouraging the algae to grow. Sunlight that comes through a window doesn't help your turtle. You need a UV lamp for the turtle.

Also try a product likd Turtle Clean. There are other similar products in pond stores. They supply bacteria that decomposes the waste, so there is no food for the algae.

Good luck!
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0.1.0 ornate box turtle
1.0.0 eastern box turtle
1.0.0 Yellow belly slider
0.1.0 Red belly cooter
0.1.0 Australian shepard
1.11.0 chickens
1.0.0 Dutch(rabbit)
3.2.0 children (do I still count the married ones?)
1.0.0 husband