Hello EvilTwin and Welcome to Frag Outpost.
Congratulations on your new tank. How are your alkalinity and ph levels?
Cladiella (Blushing Leather) is a pretty hardy coral and should do fine under most lighting conditions, but will probably prefer to be kept higher up when using power compacts. This coral also needs to be placed in an area of high flow.
Are you currently using any supplements or 2 part additives?
When you said that the yellow polyps just fell apart what exactly do you mean? Did the polyps just melt away or did each polyp detach itself from the rock and float away? Either way it sounds like some of your parameters are off somewhere.
Since you just set up the 75 gallon so recently I would wait at least 4-8 weeks for the tank to settle before adding any more corals. Do you have any detectable ammonia or nitrites?
How is the tang doing in the new tank? I'm sure it is much happier in the 75 than the hex.
One more question - What kind of lighting was the leather under at the store?
Thanks again for joining our community!