Brown algae is a very common and generally seen in new aquariums (those less than 4 weeks old). Whilst it may look a bit horrible, it doesn’t hang around for long and looks like the below. If left untreated, it will block visibility into your aquarium but rarely does it get that bad.
How is it caused: Too much light, too early on.
How to get rid: Really easy – reduce your lighting to 6 hours per day when your tank is new and keep up regular water changes (minimum of 30% once a week – the more the better). If you’re running a tank with 4 T5s, reduce it to 2. If you’re using a reflector/s now would be a good time to remove them. If you are using LED lighting (good choice) you need to reduce the intensity of the lights with a controller, otherwise try elevating the height of the LEDs and this will reduce their intensity.
Any brown algae on the glass, use a soft sponge and wipe the glass on a daily basis. But make sure you squeeze the sponge into a bucket of old tank water, otherwise you’ll add the algae straight back into your tank.
This algae may also cover your rocks and some plants too and if that is the case, you can clean the rocks easily (whip them out and get a toothbrush on them) but for the plants, you’ll need to use algae eating fish or shrimp.