you know how it goes, your kid is getting something and you're the one who is supposed to know the answers lol

anyone knows care of curly hair tarantula? i know the basics ( ground dwellers, humidity 70-80, temps 70-80)
problem is that the upper part of the belly has a bold spot, quite big, it got bold a few days after we got it in May, then the guy molted in june, the belly was all right for a week and then got bold again. the weird part is that the longer hair is remaining, but smaller darker hair is gone here. any ideas or suggestions?
as well how to do a terrarium for him/her, other then just go throw a few rocks/cork/etc on top of cocofiber substrate. how the tank cleaning should look like? how often?