From the Super Mario Wiki
Falling Spikes are spiked obstacles that fall from ceilings, and were introduced in Super Mario World (although the Icicle from Mario Bros. can be considered a precursor). They are found only in Castles and Fortresses, and are typically grouped together amongst standard Spikes. They are slightly off-color compared to normal Spikes. Unlike Icicles, Falling Spikes will remain on the ceiling until Mario or Luigi approach one, which will trigger it into falling, similar to a Thwomp. Once it falls, a Falling Spike will continue to fall through the floor until it disappears. They are darker in color than normal spikes.
They reappeared in the Game Boy remake of Donkey Kong and Mario vs. Donkey Kong. This time they get stuck on the floor after they fell. Mario or Mini Mario can use its flat end as a platform. After a few seconds it will disappear and regenerate in its original location.