Does Sandswept Tomb work with Moonfall?

No, Sandswept Tomb does not discount Moonfall.

Sandswept Tomb reduces spells that "choose one or more units here that are friendly to it." Moonfall chooses a battlefield, not units. Its optional move instruction can choose an enemy unit elsewhere to move — but that unit is an enemy, not friendly, and it isn't "here" at Sandswept Tomb at the time of choosing. The -2 Might debuff is a blanket effect, not a choice. None of Moonfall's actions satisfy Sandswept Tomb's condition, so no discount applies.