Post a good picture showing the entire tank and a couple of pictures of the fish. Make sure they are in focus and we can see the fish.
From the current picture it looks like you have blue green algae (Cyanobacteria) in the tank and that would suggest excess nutrients in the water and substrate.
Do you gravel clean the substrate when you do a water change?
When was the last time the filter was cleaned?
How do you clean the filter?
How much water do you normally change?
-----------------
Wipe the inside of the glass with a clean fish sponge.
Do a 75% water change and gravel clean the substrate every day for 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. 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.
Make sure nobody is using sprays, perfumes, deodorants, smoking, painting or doing anything else in the room that produces fumes.
Make sure any buckets you use are not used for anything except the fish tank.