I dunno. He doesn't seem to fit the characteristics of a Mustard Gas. Here is what I found....
The original Mustard gas bettas had a solid blue/greenish body and yellow fins. Because this line bred true Jude Als thought of this strain name, but actually there not more then bicolor fishes.
Unfortunately nowadays, the term mustard gas is a very misused term. The pattern of the colors of the different MGs differ from fish to fish. Some fish also have blue/green/black edges around their fins while others lack these features. Most of the "modern" Mustard Gas don't even come close to the original of Jude Als.
Because of these controversy, at shows, mustard gas bettas are classified into the bicolored class.
Chocolate Bettas as they are commonly called, have a brown body with yellow fins. But actually they are black/Yellow bicolor bettas.
Chocolates are often bred to yellow in order to improve the color of both lines.