They might be worm castings. They look similar to the ones in this picture. Maybe check the soil to see if you can find any worms, if you're concerned. They shouldn't harm the plants.
Alternatively, they could be from caterpillars, but it looks a little wet for caterpillars to want to venture there.
This is, just as you thought, a mould, which lives off the organic matter in the soil. Remove it and provide more air circulation if possible. Stirring up the top half inch of the soil sometimes helps too
Can't find mention of anything new affecting this particular tree; you may have already read this particular link, but according to this https://web.extension.illinois.edu/hortanswers/plantdetail.cfm?PlantID=214&PlantTypeID=7, because this tree is widely planted, some of the problems it's prone to are now considered serious.