Frozen
Ice has locked one of your blocks. You can't match it, but you can still swap it out of your way, and it thaws on its own over time.
In detail
A frozen block cannot be matched, and matches forming around it treat its cell as empty. The duration is set by the enemy that placed it, and the block thaws on its own when the timer reaches zero. Freezing follows the block through swaps and falls. Abilities that clear cells outright, such as Chain Lightning, still clear a frozen block. Enemies place it two ways: an aura drips new frozen blocks on the enemy attack interval, and on-hit frost rolls separately on each strike that connects.
More in glossary
From the current development build. Content and balance may change before release. This public reference includes discoveries and does not reflect your personal unlocks.