To store a carpenter's square, nail either a 13-in. or 22-in. length of tongue-and-groove flooring to your shop wall, groove side up. Then, to hang your square, just drop one leg of the square into the groove of the flooring piece. The length depends on which leg you want to be horizontal.
Chlorine bleach is excellent for removal of mold, mildew and algae, but it also can cause premature failure of the coating. Try using a non-chlorine bleach cleaner instead.