I just found a marsh which was blue-green on the minimap. It wasn't quite the same as usual. When I investigated there were no logs, algae clumps or other features and I didn't pick up any leeches.
My guess is that the type of marsh which used to appear white on the minimap is now showing blue-green. It's not quite the same as the leech marshes. This one has some grey marbling streaks in it on the minimap, similar to those found on the purple marsh and some yellow-green marbling.
I don't know if this is a one off, a change to all previously white marshes, a third type, or only found on the newly generated grids.
FYI