Yeah, that white piece is the seal. It looks to be installed improperly or has shifted into a strange position.
The seal could still be usable, you just need to make sure it is level with that brass cog-like piece below it.
To re-seat it, you'll have to take apart the silver rod from that brass cog-like piece. But you want to make sure you don't damage either part while you do that.
When I re-installed my valve seal I believe I heated up that piece with a hair dryer first. I put a couple blocks of wood in a vice and clamped the brass piece in between the wood (to not damage it) and then I took a rubber inner tube and wrapped it around the silver rod there (to not damage it) and used a wrench clamped over the rubber to unscrew it from the brass piece.
Also, that spaceship looking thing on the left of the picture looks like it needs a good cleaning. The ridge of the hole in the center of that spaceship looking thing facing the camera needs to be clean, flat and unmolested. That ridge on that piece contacts the seal and is also important in preventing that barrel leak.