Its not a great photo, but black beard algae?
Reduce the lighting period or intensity.
Reduce available nutrients, ensure you arent overstocked, overfeeding and up the water changes.
If you can remove the infected plants, decorations etc you can dip them in a 3% hydrogen peroxide solution for 3 minutes, thoroughly rinse and return to the tank.
An overdose of seachem excel can kill it off, but this is risky, so thoroughly reaearch this. Ive spot treated BBA with excel using a syringe, which is much safer but only really useful for small patches.
Ive seen it claimed that nerite snails will eat it, but mine never have. They also didnt touch diatoms which everyone says they eat, so maybe i just got a batch of picky eaters.