We noticed yesterday that this mob does not count towards the Mash Brownie Quest. Not a big deal as its a rare spawn though I thought it was worth mentioning.
Strange that it does count as a brownie when it comes to killing off brownies for the quest.
The Captain of the Leafguard seems to be a rare spawn. Combined with the location in the far south west part of zone, its probable that not many folks venture in that area. Our group only spotted him once yesterday after working several of the Rescue and Mash Brownies quests. Also, it seems this mob only wanders the higher areas as we've never seen him on the ground. We have spent many an hour on the ground working faction and never saw him once in over several hundred fights.
I hope this provides some good documentation.
p.s. We saw the mob at the top of the ramp leading up to the Queen Veneneu and quest npc Agilica area.
The Captain seems to spawn anywhere on the bridges. I have seen him near both Leafrot and Gillipuzz on the bridges. Not much harder than the other brownies either.