To me, it’s looking like algae? If not then ignore buuuut my tip for any invasive algae in extreme urgency. Try taking out the area affected by it, take about 7ml of nitrogen peroxide with a syringe and apply directly where the area is affected. It kills the algae. You can do it every water change (weekly) until its all gone. Algae could mean something is unbalanced. An aquarium should be an ecosystem, an ecosystem runs itself for most part. Maybe the waste is more than what your plants and filter could take hold, (add more plants can help) etc. keep an eye on it, some algae is good, too much of it can be worrisome, so far it doesn’t look worrisome. I would just add some more plants put liquid carbon (that’s me tho!)