It's used to sting other encroaching corals. The specialized tentacle is packed with nematocyst, it is a defense mechanism not used to acquire nutrition.
A little fact: The sweeper tentacles of Euphylliids are some of the most dangerous among commonly kept corals. Aquarists should exercise due caution that they do not get stung and tankmates will need to be placed at sufficient distance. Never underestimate the reach of a Euphylliid's sweeper tentacles. Sweepers 10 inches long are not uncommon and larger ones have been observed. Caution with such tentacles must be exercised or aquarists will suffer some significant degree of pain from the "right tentacle on the wrong day."