Having to need high heat , long hours of light , and water it's a party ground for algae having a turtle ! You could have all lights on a timer ( on 10 hours a day ) for a day/night cycle. Adjust your maintenance schedule. Watch very large water changes that causes algae blooms too. Smaller (%) more often is better and will help keep a cycle going if your tank is cycled. Also to me a simple fix but works is // start with a clean tank and add live plants ! Floating plants works better , a turtle will dig up rooted ones just to see what's under it.
Goggle for safe plants or read this :
http://www.redearslider.com/plants.htmlSome plants are better at producing oxygen and others better on controlling algae. I think floating water lettuce controls it well and hornwort ( just let the hornwort float) of tie it to something works the best for my setup. Each setup is different and would need different amount of plants.
The live plants will out compete the algae for the nutrients in the water and also give the turtle some things to pick on while at work and or play in. You take the food supply away from the algae it dies. You can Not remove the heat , light or water so remove its food source. Also you can add a few ghost shrimp they are cheap and do eat some algae without adding lots of waste as a cleaner catfish would defeating the purpose of having a catfish. Yes with a turtle they will become food but he has to hunt for them and they are cheap. Will give your turtle some exercise and let it be a turtle hunting ! By me ghost shrimp are 10 for a dollar but I keep a DIY trap in a lake and catch mostly my own free !
The algae on shell double check basking area for correct temps so it bask more , that will take care of algae on the shell. Do Not use or try chemicals to remove the algae , long time turtle keepers try to keep chemicals out our tanks for control.