Hi and welcome to the forum
Anubias plants are marsh or garden plants and don't grow fast when kept underwater. They are also grown out of water and have terrestrial leaves when you buy them. These leaves die when kept submerged and start to break down and rot away. New smaller leaves usually grow in their place.
Basically you have a plant that was grown out of water and is now losing its terrestrial leaves.
If you have brown spots on leaves and it rubs off easily, it is usually algae. A mystery snail or small algae eating catfish will normally clean it up. However, make sure the water chemistry (GH
& pH) and tank size and tank mates are suitable for the catfish.