Sweepers are the tentacles that come out of the coral. Since they are kind of used in defense (or offense), they're called sweepers because they sweep back and forth preventing neighbouring corals from growing too close (or outright killing the neighbouring corals). These tentacles tend to be much longer than the tentacles used for feeding. Some corals are quite aggessive with their sweepers. Bubble corals, pavona, hydnophora, are some examples of corals with big sweepers, and need to be carefully placed in the tank, so as not to accidentally sting other corals. Hope this helps!
