This is in a 5.5 gallon tank that's been set up since the end of November. It started a couple weeks ago and the spots are growing in numbers. It's near the middle of the tank and is mostly on this plant. All the plants are silk. There's a small spot or two on the dark purple silk plant next to it and a bigger spot on the rainbow cave on the side closest to this plant and also on another coal cave decoration, also on the side closest to this tank. It looks like this brown stuff dripped all over the plant. It scratches off of the hard decorations easily like dust. It looks dusty, sort of.
The tank is at 80 degrees and houses my blind male CT. It is filtered but not yet cycled and gets two 50% water changes a week with Prime, VitaChem, and Epsom salt at 1tsp per gallon for his chronic bloat and buoyancy issues. The lighting is just a very small LED light but the entire set-up was brand new when it was set up in November and it doesn't have natural light from a window hit it and all the plants are silk. Also, since he's blind, a lot of the time his food will sink before he finds it. This is the only tank that has it.
I don't know if all of that is relevant but I want to know what this is and if it could harm my Betta.
Edit: I forgot to mention the lights are on a light timer and are on 10 hours a day. The plants are all from Petco. I looked at threads of others who had brown spots on silk plants but the spots on mine look to be darker and more like something's dripped on them, but the tank is covered and I don't see anything on the lid 0.o