Wow it’s in quite a state! I had an indoor cage like that when I adopted my two, they came with it. Mine was clean though, i’ve moved them to a bigger c&c but while they were in it I used to clean it with hot water and washing up liquid (well rinsed). If you leave it to soak I’d imagine that it would come off with a good scrub. Might take a couple of attempts though. Good luck!