Hi and welcome to the forum
It could be excess mucous, a fungal infection or a protozoan infection.
Check the water quality for ammonia, nitrite, nitrate & pH.
If you have any ammonia or nitrite readings, or a nitrate reading above 20ppm, do a 75% water change and gravel clean the substrate every day until the levels are 0.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.
If it spreads over the next few days post the water quality readings and another picture of the fish and we can look into medication.
Monitor the fish and see if it rubs on any objects, that would indicate a protozoan infection or poor water quality.