So every Autumn around mid-September it becomes a red crescent, and that's just a trivial accident?
I suppose having an aerial cross, or flag, or 93 was too simple an idea.
As strange as this red crescent is as a design for this memorial, I think it is most likely accidental. Paul Murdoch Architects is a small firm with no obvious links to Islam and the design was approved by a board that consists of family members of those that died in flight 93.
That said, it is still one of the odd coincidences I can imagine. And it will probably be changed once the connection to Islam is pointed out to the families.