What happens if I Moonfall a Scuttle Crab?

The Scuttle Crab's Might becomes -2 (treated as 0 for most purposes), and it survives. Moonfall doesn't deal damage, so the Crab isn't killed by the reduction alone.

Per Rule 143.2.b, Might can go below 0 mathematically. Scuttle Crab (0 Might) gets -2 from Moonfall, making its actual Might -2. However, for combat damage assignment and any spell/ability that references its Might, it's treated as 0. And since Moonfall doesn't mark damage, there's no lethal damage condition to trigger a kill.

The Crab stays on the battlefield with its abilities intact (it can still conquer and hold since its rules text grants that regardless of Might). If you later applied a +1 Might buff, it would calculate as -2 + 1 = -1 (still treated as 0), and a +2 buff would make it 0 (treated as 0).