If your turtles are growing algae on their shells, inspect them thoroughly once a week to make sure you are not missing any potential problems. (You should be doing this anyway!) Algae also have beneficial effects: they help keep the water clean and healthy! I usually let them grow a bit in the tank for a less sterile appearance. In order to grow, algae need some sunlight and nutrients. Deprive them of both, and your problem will lessen or go away. Also, keeping the water moving will also make it harder for the alge to take hold. To prevent algae growing on a turtle, change the water frequently, and brush off any algae growing in the tank. Don't expose the tank to too much sun. Adding a bit of aquarium salt to the water every other week, to change the composition of the water, can also prevent algae growth. Aquarium stores sell chemicals to kill algae. These are not recommended. To remove algae from the turtle, brush it with a SOFT brush. A discarded toothbrush works really well. Usually, not all algae will come off the first time. There are many different kinds of algae. Some grow in clean, healthy water, and some grow in muck. As a general guideline: Algae that look dark green and that grow in carpets or patches are fine. Long, stringy, slimy algae are not. If you get the latter kind, change the water more often or get a bigger filter. If you notice the shell of your turtle flaking or pieces coming off-he is shedding. This is normal, and he needs a sulpha bath to aide in "leftover" removal. However, if the shell gets soft or rancid smelling spots on it-this is shell rot. This needs to be attended to immediately! Shell rot is caused by a fungus or bacteria infection. It is very serious, and if left untreated, can lead to other diseases and infections. The infection will begin if fungus or bacteria is allowed to enter cracks in the shell. Signs of shell rot include discoloration beneath the surface of the shell, soft spots on the shell, and a foul smelling discharge around the soft spots. To treat this yourself, remove the thin layer of peeling keratin from the affected area, then gently clean the bone with betadine cleanser or povidone-iodine, and apply Gentocin or Polysporin ointment. Repeat for three days. Then paint the affected area with 2% gentian violet every other day until it has dried up. Protect the area with a bandage so it stays clean, but make sure air can get to the wound. Keep the turtle out of water except for necessary soaking and to eat. If the area of shell rot is large, the animal should be taken to a veterinarian that treats reptiles. To prevent shell rot, fungal infections (white or gray spots or tufts on skin) should be treated as soon as they are observed. They can be treated with OTC fungal medications that are used for tropical fish in aquariums. Clean and disinfect any cuts on the turtle's shell, and apply triple antibiotic cream immediately. The enclosure should be kept as clean as possible, and all feces and uneaten food should be removed at least daily. The water should be changed daily, or sooner if soiled. Hope this helps your little friend and you. Good luck!

My turtle tank usually grows green algae when I leave the UV light on for too long, but unless it really bothers you it is not a problem.

try scraping it off

I recommend using a toothbrush with soap and try scrubbing the shell gently with not much force. Its algae and normal for it to grow. Remember that cleanse is very important for its health.