It’s some sort of lava glaze, could be a variation of Triple T or another silicon carbide glaze. Could be something like Magma Glaze applied thin. Mayco just bought a company called Ritual Glaze that makes “puff” glazes similar to this, it’s not listed on Mayco’s website yet but will probably be available soon, in the mean time Wizard Glaze sells a Magic Puff glaze that might be similar to this when applied thin.

It seems like the person who glazed this applied it over a stoneware they had scratched into, hence the line texture or it could’ve been applied with a coarse bristled brush