A very powerful filter is the next step, preferably a canister type filter (a pet store can help you choose one) rated for a tank 3-4 times the size of your actual turtle tank (turtles really are messy and need the extra filtration). Feeding the turtle in a separate container can also help reduce the amount of waste in the water that can lead to algae growth. When you clean the filter, don't disinfect it or use really hot water to clean it, because you don't want to kill the beneficial bacteria it harbors (which are needed to degrade the turtles' waste products).
Make sure the lights on your turtle tank are not on for too long. The lights should be on for 12 hours or so a day; if they are on longer they may contribute to algae growth (not to mention stressing your turtle). Don't be tempted to cut the lighting too much though -- while the lights do contribute to algae growth, proper lighting (both basking and UVA/UVB) is critical to your turtle's health. However, if the tank receives any direct sunlight, moving it out of the sunlight can help a lot.
Keeping the tank clean is vital to water quality and will help with the algae (see Keeping your Turtle Tank Clean for help). Keep in mind there may always be some algae in the tank (and perhaps even on your turtle's shell) -- trying to totally remove algae is futile and unecessary; the goal is to keep the algae under control, but more importantly, make sure the water quality is good.