Relative Surface measure of green algae in greenish turtle shell

[alex_mauremys]

Dear friends,
I am new ImageJ user and I have a problem with make a measure of relative surface of a turtle shell... I have photos in JPG and I have to measure the algae surface in the shell, the problem is that the color differences between turtle shell and algae are very little. How I can do it automatically??

Thank you very much

P.D: I can upload photos