It could be fungus or excess mucous. Check your water quality. Do a big water change and gravel clean the substrate. Add some salt. If there's no improvement after a couple of days with salt, post more pictures.
Check the water quality for ammonia, nitrite, nitrate and pH.
Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate.
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. Wash filter media/ materials in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn.
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Add 2 heaped tablespoons of rock salt, sea salt, or swimming pool salt for every 20 litres (5 gallons) of tank water. Keep the salt in the tank for 2 weeks then stop using salt.
If you do water changes while using salt, add salt to the new water before adding it to the tank so the salinity (salt level) in the tank remains stable.
