Algae is 100% safe for turtles. It is a result of either too much direct sunlight, too much nutrients in the water or a mix of the both. If the tank gets direct sunlight each day maybe try covering the side that sees the sun with thick paper or a towel. If you are feeding your turtle in the tank, begin taking the turtle out and into a container of tank water for feeds and return it to the tank once it has eaten. Any uneated food can then get washed down the drain and not used to feed algae.
Even if you don't find a way to rid the tank of algae, the turtles will eat it as it grows.