That looks like a 3" or 4" square block of rock wool. It looks like it has way to much water so it grew algae growing on top. You need to at least cover the grow medium from light until the plants leaf structure can provide coverage.

Now the big question you need to check the Ph of the rock wool cube. I bet it will not be 5.2 - 5.5. Rock wool has a tendency to raise the Ph and with the alga growing that too will affect the Ph of the rock wool. That's one of the reasons I quit using rock wool several years ago.


Good luck