Mold will grow in tropical enviroments it just the nature of things. You could seed the enclosure with isopods to help control the mold spores, but you will have to get tropical isopods. They live in environments with 100% humidity which will be inside the substrate and under the water dish. I am unsure how well they will reproduce in a Pacman enclosure though since dart frog enclosures are vivariums that are actual pieces of what the animal's natural habitat is. Sometimes a little extra ventilation helps, but the moisture levels in your tank are not excessive.

Scoop the mold out. Mix the substrate up once a week. This will reduce mold growth.