Hi, walamma.
I use the Gunze "Hobby Color" acrylic clear flat all the time, thinned with water and sprayed with a Tamiya HG Trigger AB, with excellent results.
You say the clear flat you used was "Mr. Color," right? That's Gunze's lacquer line. "Hobby Color" is their water-based acrylic line. If you thinned "Mr. Color " with water, I reckon you would get some kinda funky finish! But you said it indicated using water for thinning, so, hmmmmm.
I'd check all my labels and make sure which one you have, and try it again!
The only frosting I've ever experienced with a clear flat coat was when I used Gunze's Mr. Super Clear lacquer flat clear on top of an enamel finish that had been washed and weathered with acrylics. I was surprised, but I actually liked the effect. Made a tank look really dusty, and dead flat. So I didn't bother trying to "fix" it at all. Now I use it on purpose to get that dusty look!
What had happened was the lacquer clear is not intened for use on top of anything but lacquer finishes. It says right on the can (which I read after the first application!), "Do not apply over water-based finishes or decals." I don't know what other adverse effects it could have, but luckily I'm not a heavy sprayer, or perhaps the whole finish would've melted off!
Anyway, check the labels, experiment with different ratios and application densities, and try it out on something expendible first!
Good luck!