As Ed said, wet substrate is a great breeding ground for bacteria, fungi, and all other kinds of ill stuff that aren't good for your snake.

Make sure that the hides you are using, or should be using if you aren't, in the enclosure are secure. Snakes don't "play" in the mud...chances are it saw the bowl of wet substrate as a better hiding place than it had before, and thats why it went in there.

So, specifically:

My question is should I replace the aspen with moss (softer). And should I clean up the wet aspen that is around the slush bowl?
No, wet moss does not make a good substrate for ball pythons. Their enclosure should be HUMID, not WET....there's a difference.

Is there a high probability of an RI from this set up?
So long as your temps are where they should be (82-84/92-94) and your humidity is between 50-70%, you shouldn't worry too much.