green slim on pond

Hi Gizmo, well, it's an indication that there's too much nutrients in the water making it ideal for the algae/weed to grow. Where is the pond placed?, does it have any Fish in it?, what do you do to maintain the pond? I have a approximately 22' x 12' x 3' 6" deep wildlife pond which I dug in 93' and, I can say in all honesty, has always had crystal clear water despite having Fish, Frogs and other wildlife in it without having to resort to filters or pumps.
Smaller ponds are more difficult to maintain but you have done the right thing by placing Marginal Plants in it as they take nutrients out of the water as they grow and my pond has a lot of them in it along with a linked Bog Garden. How long have you had the Marginal plants in the pond?
You need to cut down the amount of nutrients in the pond water to deny the algae/weed the ability to grow so if you're adding anything into the water, that might breakdown into nutrients, that needs to be reduced or stopped. Algae/Weed, like other plants, also needs sunshine so a little light shading if possible might help.

 

I used my pond as an example to show how fairly easy it is to keep the nutrients down by natural means to the minimal, pattie, by using enough marginal plants to take up the nutrients So, again, the problem is, whether the pond is inside or outside, that there is too much Nitrogen in the water.