Yes, thanks for the pictures. Your betta doesn't really fall into a specific morph, it appears he has had alot of things bred into him. The only thing that he really has is a black head which is what breeders call the mask.
I would personally consider that to be a low grade mustard gas....
As for fin, it does look like its a delta, but it could also be a halfmoon. The only way to check is to get it to flare, if its a D shape then its halfmoon, anything else (aka a triangle) and its a delta.
He seems pretty chill. I only saw him flare at the shop. After watching him for a bit, it doesn't look like he has a complete half moon. So as much as I would love for him to be a halfmoon, I think he's a delta. After looking at some pictures of Mustard Gas, he does look like a paler version of them.
I agree that there was a mustard gas bred into him at once, but i don't think he is a high enough percentage to be called mustard gas. Although he looks like a low grade, he has too much impurites of blue on his fin to be called one.