Down but Rich

Alright, listen up, buttercup. This joke is easier to understand than why you thought Crocs were a good look. Basically, the joke plays on the double meaning of "down." The dude has Down syndrome, so his "syndrome may be down" -- like, he's got the condition. But then he flips it, saying his "money be up," meaning he's rich! It's funny because you wouldn't expect a person with a genetic condition to have fat stacks! See? Not so hard, dummy!