Those are just his markings.

Everything in your setup looks OK - You have the UV nice and low (What kind is it and how old is it), you have created a nice basking area (Although I would check the temp underneath that because depending what wattage bulb that is, it could be a little too hot on that tallest branch), lots of things for him to climb on - apart from one thing......

Sand shouldn't be used as a substrate for babies as it can cause impaction if ingested. I however don't even use it for adults (It's unnatural, as they originate from areas of clay, stone and compacted sand, it's smelly, unhygienic and expensive long term)

What most people us is newspaper. It doesn't look that great, but the safety of your pet should be above the aesthetics of the vivarium. You could also use paper towel - but newspaper is sometimes free and you will be using a lot of it.

Some people decide that when they are adults to transfer them to childrens play sand.
However, I use floor tiles. They look great, are easy to clean, were really cheap and never need replacing/changing, so work out very cost effective, and most important, zero chance of impaction.