Odour in hammy's ball after using

[Britishhammy]

Hi! I've got a 10 month old Syrian male (roughly, he was a rescue). Over the past 2 days I've noticed a musty unpleasant smell in his ball when I take him out of it. He's used this ball for past 4 months no problem. His cage doesn't smell and when I get him back out later, he also doesn't smell. He's otherwise eating and drinking normal, usual behaviours and seems happy. I did notice a little wetness around tail and after freaking out initially, realise he didn't have any other symptoms of this. Does anyone know what this is? Thanks