Turtles growing moss and algae on their shells in the wild are extremely common. They will spend a lot of time in shallow water sunning themselves (They are cold blooded reptiles after all!) and algae and moss love those conditions. It is less common in a tank however. I am presuming that you must have your tank close to a window which receives a lot of sunlight. The algae will not hurt your turtle, but it certainly is not aesthetically pleasing either. I have two suggestions. One is to move the tank away from the window. I bought several halogen desk lamps that have a swiveling head (Up and down, not left to right) and have them hanging over the some rocks which sit out of the water on my tank. This allows the turtle to sun itself without as much risk of algae. The downside to this is that the turtle will not receive the full spectrum that the sun gives. So you have to supplement its diet with vitamin D enriched foods. There are bulbs you can buy which claim to give off the same spectrum, but I don’t trust them personally. The other option is to buy a whack of feeder fish. Turtles will often sleep with part of their shell submerged. Once the feeder fish are a little more comfortable in the tank, they will often eat the algae right off the turtle’s shell while it sleeps.