That is a known issue of ExtraBiomesXL. It crops up in lava lakes as well - quite weird to mine around at the bottom of the map, only to stumble on a lava lake with a checkerboard of grass blocks in it, plants growing on top!
You can fix the issue by removing the offending biome: set B:marsh.enablegeneration=false in the extrabiomes config. That way you still have all the other new biomes, but no longer any issues with this one.
Also a known issue. Redpower World really, really doesn't get along with anything else that modifies terrain gen. It spawns volcanoes in the middle of ExtraBiomesXL's redwood trees, runs afoul of Twilight Forest's specific terrain features in several ways at once, isn't entirely friendly with Mystcraft either and on occasion even screws with Thaumcraft 3's world feature placement. The reason is that Redpower World waits until everyone else has finished their terrain gen, then copy&pastes volcanoes haphazardly across the landscape, no matter what was there before. It also looks where a lot of smooth stone is exposed to air (for example inside Twilight Forest mounds), and replaces whole chunks of it with marble.
This wasn't an issue in the past, because few mods did as much as Redpower did back in the day. But then the 1.3 update came along and completely ruined Redpower - the mod got hit far harder than many others precisely because it did so much more than many others. And in the time it took Eloraam to completely rewrite it, a whole lot of new mods with terrain modification appeared on the scene. I cannot really imagine what's on Eloraam's mind, but I'm sure a perfectionist like her won't be happy with the way Redpower World currently works. I'm certainly keeping my fingers crossed for further development in that area.