Tube creature

Tube creatures are large, flying oblong shapes that appear in the Experimental World in Mario Artist: Polygon Studio. When approached by the player, a warning sign appears on screen and the creature slowly moves toward the player's creation while making an odd, guttural mumbling sound. If the tube creature makes contact, it will eat one of the creation's parts, preventing the player from using it until the next time they enter the Experimental World.

According to the in-game hints, tube creatures are a protected species and attacking them is forbidden (not that the player has any means of attack).