Falling Current is a Normal Spell Card.
Spell Cards are usually activated from the hand on your turn unless their type gives another timing window.
On the current TCG and OCG Forbidden & Limited lists tracked here, Falling Current is Unlimited.
YGOPRODeck lists Falling Current for these formats: Duel Links, Common Charity, TCG, OCG, Master Duel.
A listed print of Falling Current appears in Galactic Overlord (GAOV-EN053) at Common rarity.
Among markets shown on this page, the lowest listed snapshot for Falling Current is about $0.05 on TCGPlayer (prices change frequently).
