substrate

I was moving some substrate around tonight since I inadvertently angled my gyro the wrong way and found these just under the surface of the substrate. Does anyone know what they are and if they are good or bad?

So Colby and I are getting a new 16G Coraline biocube to have as an "emergency tank" with mostly corals in it, and just a couple of fish. We are debating what substrate to use and considering using the black substrate that we see some people using (not sure what it is made of). What is your...