Hi mikey and Welcome to TFF!
The brown stuff you are seeing is almost certainly Diatoms, the name for brown algae. It is not bacteria. The beneficial bacteria you will see us talking about so much here in the fishless and fish-in cycling threads in our beginners section are sometimes seen as a brown stain on the media inside the filter but rarely seen otherwise and even then it is difficult to tell from debris in different stages of breaking down. The brown algae is probably happening to you because you are leaving your lights on too long. You can limit them to 6 or 8 hours if you've been doing something longer like 10 or 12 hours. The only true lower limit is 4 hours, which is the minimum that live aquarium plants need to make some useful sugars. It's particularly easy to get algae during the first months when a tank is "cycling" because it is ammonia plus light that triggers algae spores. Ammonia is usually present in higher concentrations during the cycling period, until the "biofilter" is fully established.
And indeed, the fact that you are showing 0.25ppm ammonia may be a sign that you need more instruction on gravel-clean-water-changes (which should be done weekly) or that your biofilter is not yet fully created and you are still in a fish-in cycling situation. Or it could be that you have ammonia coming in through your tap water and that is making it a bit more difficult to stay in the safe range until the filter is fully cycled. The three main articles to be sure to read for background are in our Beginners Resource Center: They are The Nitrogen Cycle, The Fishless Cycle and The Fish-In Cycle.
If your tap water has zero ammonia then it would be good to perform an immediate 50 to 75% water change (with good technique) since at 0.25ppm you are at the upper limit of the safe range. Although it can vary with pH, we like to use 0.25ppm as the top because beyond that you can begin to get permanent gill damage but the fish won't show symptoms of stress like they will begin to do at 0.50 or 1ppm levels. It may also be that you need a gentle squeeze of your sponge or a swish of your loose media in old tank water (the water siphoned out by the big water change is what's used) to help remove more of the debris that is generating the ammonia. Overfeeding can also help create these problems (only feed fish what they can consume in 2 to 3 minutes once a day.)
~~waterdrop~~
