You've run inside a Mad Jelly.
Mad Jelly is a jelly-like enemy featured in Majora's Mask. It serves as the Mini-Boss of the Great Bay Temple along with the second Gekko. Mad Jelly cannot move on its own, but if Link touches it, it deals damage. In the mini-boss battle, the Gekko throws Mad Jelly at Link. If the Gekko is damaged, he will collect all of the Mad Jelly into a giant glob and surround himself with it, climbing onto the ceiling. Link can freeze the Mad Jelly with the Gekko inside, causing them to crash to the floor. If Link does not freeze the jelly, it will drop onto him and trap him inside of the blob, leaving him helpless for the Gekko to attack, which ends up doing two full heart containers of damage each time. After the Gekko is defeated, the Mad Jelly melts and turns into the Boss Key, and the Gekko shrinks to become a member of the Frog Choir, who Link must speak to wearing the Don Gero's Mask as part of a side quest.