I guess the deal here is Reimu told her child not to go to that water place, but when Maricha was running away from whatever that thing was, she forgot and the rest is history.
There's no corpse here. Just a Nitori waiting to attack and suck Marisa's paste, just like in the legend told by Reimu.
Okay, thanks for that, mister Yuno. I don't know why, but when I first saw it, that thing looked like a corpse, with a "marble" of Nitori's hair tie serving as its eyes, while her braid is a tongue and the opposite "marble" is shit, you know, some yukkuri corpses were depicted with shit behind its anyaru and the body was blackened with a tongue sticking out, so I think this is one of them, LOL.