there's a question mark below the blue area that I can't access at the bottom of the cave
Hi, by myself I wasn't able to reach it.
I read something about placing an Air Plate somewhere, but I'm not sure where and not sure if it's really the Air Plate.
What I can tell you is that, if you put a Water Plate down there, you'll flood that room above the water and you'll no longer be able to collect the item.
I know because I was dumb enough to do it.