Have made a little animation on my computer and saved the file “iStopMotion Clip 2.mov” but Quicktime will not open it, which seems strange as .mov is a native Quicktime format.
I am using Quicktime 10.4 and my OS is Yosemite 10.10.2

If you want to to play the movie in QuickTime, we suggest exporting it first. While iStopMotion saves a native QT Movie, it has uncompressed data and QuickTime X will not play it natively. (QT 7 does)
Open the file in iStopMotion, Choose File > Export and create e.g. a h.264 movie.