[What is this black stuff?]

It's not really fluffy or fuzzy at all.  It's pretty smooth.  I've been searching for what it could be and came across black beard algae, but that doesn't really look like what is in my tank.

[What is this black stuff?]

I have a 29 gallon tank in my preschool classroom that had been set up for around 4 years.  A couple of months ago I had to replace the tank because it got a crack in the glass (preschool mishaps) and decided to change out the substrate and add some more plants while I was moving everything.  A little while after moving everything into the new tank I noticed this black stuff all over the place.  It seems to be concentrated mostly on the edges of the plants (particularly the anubias), but it's also on the substrate and decorations.  I'm not sure if it's algae or something else.  I tried removing it by hand, but it wouldn't come off.  My otocinclus and pond snails don't seem to be making a dent in it either.  The new growth on the plants doesn't have any black on them. 

So my questions are: What is this black stuff and how do I get rid of it?