Slime mold.

Thank you for your reply. My grandson was very curious about it so I said I would find out.

Slime molds are really neat! I've read about them as examples of self-organization. "Many slime molds, mainly the "cellular" slime molds, do not spend most
of their time in this state. As long as food is abundant, these slime
molds exist as single-celled organisms. When food is in short supply,
many of these single-celled organisms will congregate and start moving
as a single body." Your grandson might enjoy learning more about them: https://en.wikipedia.org/wiki/Slime_mold