Do you know where the mold comes from and why it forms - does it eat the
acorn?

Mold spores are everywhere. They are in the air and land on everything. If you
look in a very damp spot on the wall and see black spots, those are the spore
containers of the fungus.
As you have seen, some fungi grow right in water. When the spores land on a
likely spot, they sprout and start growing. The fuzzy hairs that you see are
called hyphae. That is the fungus's growing stage. You might say that the
fungus eats the acorn. It breaks down dead plant tissue (and other substances,
like paint and fingernails). Actually, fungi are providing a very important
service by doing this, when they do it in the right place. The ones that attack
live tissue are dangerous.
Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)