From the Super Mario Wiki
Blokkabloks are enemies appearing in Super Mario 3D Land. They have pointed green heads with angry eyes that look similar to Bullet Bills and spikes. Their tails are also green and spiked. A Blokkablok's body is composed of Brick Blocks, ? Blocks, or a mixture of the two. The top of these blocks are spiked. Blokkabloks can have between one to seven body segments.
They appear in various levels throughout the game, with noticeable differences in length. Blokkabloks do not attack Mario or Luigi, but simply hover overhead in a set route. They can easily be avoided by just walking underneath them. Destroying all the Brick Blocks and hitting all the ? Blocks on a Blokkablok will cause it to fall apart, defeating it. Coins or power-ups can be obtained by hitting the ? Blocks contained in a Blokkablok.
Small Mario cannot break brick blocks, but he can break the blocks from Blokkablok.