Off the top of my head ~
1. Illness always gets the weakest fish first, meaning the mollies were weaker than the cat.
2. Fish being in stress can lead to ICK. Whatever was stressing the mollies was not stressing the cat.
3. You could not see ICK on the cat because he is albino
4. Ick moves faster at warmer temps. The Mollies swim ant the top, the cat at the bottom.It is possible that the bottom of the tank or inside of decor is slightly cooler.
5. the cat is harassing and or attempting to eat the mollies causing them stress contributing to ICK of the mollies, yet the cat is enjoying himself and is not stressed.