meant to include this:
if it is a pervasive blue-green algae, your best bet is to remove him, clean his shell, and shock the water in his tank!
now, the difference in green algae and blue-green algae may be something that is more apparent to one more accustomed to keeping fishtanks,
but in brief, green algae is truly green in color, and grows lightly and evenly throughout the tank. blue-green algae grows in an imposing 'mat' over any and everything, and quickly robs the water of oxygen and minerals, and attacks the natural slime-coat in a healthy water environment. it is (of course) bluish-green in color. but without being familiar with both, it can be hard to distinguish the two just on color.