Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day for the next 2 weeks.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash filter materials/ media in a bucket of tank water.
If the spot gets bigger or changes colour, post another picture asap and we can look into medications. It might be fungus