Which '!' do you refer to. Can you take a screenshort to me?
But if you mean these question mark.
Yeou need to have water plate which drop from Mimic in previous level. Then, place it here at middle-top of the map.
You need to place air plate here (red circle) then back to question mark in green circle and swim to '!'.Do you have any half plate yet? There are 2 half plate to collect
> Left-half from 2nd lv. Dungeon at 2,3. Then, go back to Rat king and talk about a half plate with Tao.
> Right-half from talking with Rat king about half plate.
> Full Air plate from combine Left and Right half plate. Check on inventory>>Misc.>> click 'use' at any half plate to combine.