Self-forming clay is activated by cards that cost your HP. Therefore, assuming the block it produces actually gets used, it will effectively negate the disadvantage of paying 3 HP, and can even turn paying 1 or 2 HP to your advantage. For example, Brutality or Combust will cause you to gain 3 block a turn.
Weekly Patch 51: Recursion
Fixed Self-Forming Clay relic triggering incorrectly off of prevented damage from Buffer power.