Sponge
From Inkipedia, the Splatoon wiki
Sponge Blocks are one of the mechanics in the Hero Mode of Splatoon.
Description
They expand when hit by squid ink, but shrink when hit by octopus ink. When expanded, you can swim in them as a squid. You can use them to cross gaps, however enemy attacks will cause them to shrink and possibly cause you to fall to your death.