Looks like it is probably going to come off in pieces. IF you try to turn it off with any kind of extra force be aware that the neck it is attached to is silver soldered to a steel tank AND is easily twisted off or cracked creating a leak and bigger problems than you now have.
It looks like you have plenty of room to cut the edges working carefully with a Dremel tool.
Wish mine was that easy to access...most are not, they must have made a mistake in not burying it.