I know the feeling! I got a 15 gallon tank going about a year and a half ago. It looked great for a few weeks. Then I noticed it was developing this fuzzy brown thing. I went away for a long weekend and when I got back, the entire tank was covered in the stuff. So of course, I totally freaked out about it. I did a water change and also picked off as much of it as I could. Then I googled, what is brown growth on plants and learned it was just harmless diatoms. I’d like to say that from then on, I would ask questions first before doing anything to the tank but it took some time and several more freakouts to learn that lesson.