First, note this from wiki:
In science, however, the term glass is usually defined in a much wider sense, including every solid that possesses a non-crystalline (i.e., amorphous) structure and that exhibits a glass transition when heated towards the liquid state. In this wider sense, glasses can be made of quite different classes of materials: metallic alloys, ionic melts, aqueous solutions, molecular liquids, and polymers.
From what I'm reading on the manufacturing process, and given the above, those little pieces scientifically would be referred to as glass, but they're not what we commonly think of as glass, as they're not mostly made of silica.
The shape and look of the pieces isn't because they're putting broken pieces of glass in or something, it's because during the melting, hardening, and processing, some of it comes out looking glass-like.
I highly doubt they're going to injure your fish as is, nor are they going to shatter in tank and turn sharp.
There are fish safe pet store substrates that have even sharper pieces, like the marine sands which have broken pieces of shells, yet sand sifting fish still seem to deal with them just fine.
A description of coal slag (more technically referred to as boiler slag, since apparently coal slag is a catch all for various products that aren't related to black diamond):
http://www.rmrc.unh.edu/tools/uguidelines/cbabs1.asp
When the molten slag comes in contact with the quenching water, the ash fractures instantly, crystallizes, and forms pellets.
If you wanna do more research, search for the more technical term, Boiler Slag.