Hello elmim1,

From the sound of your desciption I believe you have algea growing or possibly (remote) a moss, and not mold. Most molds are white, some brownish.

Not sure how harmfull it is, but since this is outdoors and I assume water/fed to waste it won't at least clog up plumbing, pumps or a rez.

Are these plants in the ground or in containers?
One possible solution would be to mix a mild solution of FOOD GRADE hydrogenperoxide and water, then spray that on your grow medium.

Maybe someone else that's seen green where it didn't belong can chime in.
Good luck.