Glass
From Inkipedia, the Splatoon wiki
Glass is a terrain feature in Splatoon.
Description
Glass is a clear surface that is ink resistant. Inklings must walk over in humanoid form, or flop over in squid form as the surface cannot have ink on it.
Use
In multiplayer mode, glass is used as a natural territory barrier so players usually transform into humanoid form to pass.
Glass can be found in the fight against the Rampaging Octowhirl to make it harder for the player to get enough ink down to win.