I took one of the greybeard wordwall quests at arcwind point, and I killed the dragon there.

I got the word, but the arrow is still pointing at the wall; now I can't get another quest from them.

Is there a fix for this?

There is no known fix for this bug, which was introduced with the Dawnguard DLC. Once you get this miscellaneous quest from Arngeir for this location, you can no longer get any more word of power locations from him. See this thread for more information: http://www.gamesas.com/topic/1426723-bugged-quest-at-arcwind-point/