I have a 55 gallon with one common goldfish in it. It’s been cycled for about 3 months. Sand substrate. No live plants. About a month ago I had to replace one of my lights and put led lights on one side of the tank. Last week I noticed brown residue on the front of the tank glass but only on the side with the led lights. I cleaned it off. This week the brown residue is back and it’s also on my plastic plants. Is this algae? And what should I do?