Anyone heard of enzymes how to??????

I am getting some yellowish and brownish stains on my fiberglass pool. I brushed but they wont lift off and I read if I used "enzyme based pool chemicals" whatever that is ....it can take care of the problem. The only steel going into the pool is the ladder and there is no stain there just on the steps. Anyone know if this? I don't seem to have algae to speak of and its nowhere near hot enough for shocking the pool, so what should is the best way to handle this. Again pool is fiberglass. Do I need to get in the pool and scrub and with what? Thanks Dave btw the steps are fiberglass molded into the pool

You can try this test for metal stain. Place a vitamin C tablet on a stain in calm water (shut pump if needed). Check under the tab in a few hours, if the stain under the tab is lifted, it is probably metal.