Cement, if you are serious about your wildlife rescue work, then you would not do anything that would risk harming or injuring the turtle.
Algae on turtles is natural and harmless. Leave it there.
Remember that turtles can feel contact on their shell. Unless you enjoy the feeling of someone holding you down and scrubbing your head with a toothbrush until you are bald, then don't do it to this turtle.
For 'informed' advice on turtles, switch to this site:
www.australianfreshwaterturtles.com.au
Regards,
Michael.