It’s algae. Nothing that is going to harm your roots or plants, however, it’s indicative of your media staying too wet - which will harm your roots and plants.
In case anybody was curious, I let my soil get very dry before the next watering which I added some hygrozyme to (I know it's on meant to treat root rot and it's more of a preventative, but it couldn't hurt right? Lol) Well my sick plant has started growing again and my two good plants are looking beautiful and ready for a topping before next watering which hopefully gives my stunted plant some time to catch up. Here's a few pics. The last one (#3) was the sick one.
Hygrozyme to the rescue. Keep using it! It will not hurt a thing! It will keep that bad bacteria from progressing. We don’t buy beneficial bacteria products for nothin!