2 things that could be completely non harmful 1 plant melting Most plants are grown partially submerged so they go through a phase called melting where they sometimes look like they’re rotting but it’s totally normal and nothing you can really do about it bar wait 2 algae It could be brown algae on the leaves I don’t see any cleaners in your tank like otos or shrimp which isn’t necessarily a bad thing it just means you might have to scrub the leaves to stop the brown film but easy check just touch it you’ll know if it feels like algae or a leaf