Walk away out of visual range and come back. It's a network issue thing (server fails to tell your client that you can see them).
Weird that it seems to happen to the most used NPCs too, neh?
____________________________I've seen things you people wouldn't believe. Airships on fire off the shoulder of Bahamut. I watched Scapula Beams glitter in the dark near the Three Mage Gate...
There's always...not trolling him?
You're new here, aren't you?