Slime Blocks, or Goo Blocks, are a type of breakable block introduced in Specter of Torment, that also returns in King of Cards. They appear in Lost City and Tower of Fate: Entrance.
Upon hitting the block from the left or right with an attack, Specter Knight and King Knight will be bounced back, opposite to the direction they are facing. If they jump on the block, they will be bounced upwards in the same fashion as if one jumps on Goo. Doing either of these actions will immediately break the block afterwards.
Ambirders are a technically variant of Slime Blocks. They appear as regular Slime Blocks but they have a Birder trapped inside, as pictured above. If the block is broken in any way, it will release the Birder which will start behaving like normal.