The Flomp is subspecies of Thwomp. The Flomps live in Bowser's Lava Lair and Grandmaster Galaxy. Flomps are floating platform enemies that like to sleep and chill out. You need to jump to next Flomp to finish the level. The Flomp is stationary and looks like a normal platform, but after a short amount of time his sleepy face turns into an evil grin, the Flomp shakes a bit and flips. If you're on the Flomp when it flips, you'll get flung back into lava, space, Black Hole, or another Flomp. Flomps can't be defeated in any way.
Being a Thwomp subspecies, Flomps are stone enemies. However, they have a green octagonal frame adorned with spikes and two screws on the sides, making it resemble a watch: this is probably referred to the fact that timing is important when dealing with Flomps.