I noticed that my Amazon swords (as well as another plant I can't remember) started getting brown blotches on the mature leaves. I figured they were getting old or not enough nutrients. Well adding nutrients for several weeks didn't solve the problem. Then I noticed the brown stuff on my glass. So I cleaned it off and realized I could clean it off the leaves. I'm guessing it is a type of algea but I've not seen one that was just a straight brown color. What could this be?