Ah ... The way I understand it, once the installer has run, it deletes itself from the hard drive. Sounds like the cleanup routine didn't finish somehow. Yeah, I would just delete that if it were me. If you still have the disk image on your hard drive, copy that to an external drive just in case you need it in the future. But if you don't, there's no reason to save the icon.

But I thought you were saying the icons for the other apps were also appearing as question marks?