Coming up from the ground

We have piles of mulch applied to the various planted areas of the apartment complex.
I love seeing the rose bushes, which fortunately were allowed to remain when other plants were removed, like the big hibiscus bush. Gone. But this week, right about where it used to be, there was a disturbance in the earth.

 Taken Sunday July 21, 2019

A bit closer...no ants are to be seen, though that was my first thought.

Then after the thunderstorm this afternoon...a big change!

Taken 6 pm Mon. July 22, 2019

It's turned black on top...not at all fuzzy like mildew...very strange thing.
do you have any ideas?