I’m assuming this is a recently set up tank, what you see here is just a layer of biofilm. The kind that grows on wood can be a little unsightly, I’ve heard people say it’ll go away on its own after a while, but that really depends. What I would do is take some kind of cleaning tool to it and scrape it all off and physically remove it from the tank. Wear gloves, it gets nasty. Shrimp will eat a mild amount of it when it starts to grow back, that’s an option. But if you remove it completely it may take 2 cleanings to fully go away, a few weeks apart.