Mike, your cheese curdled. The protein curds separated from the watery whey. In most dishes (mac & cheese especially), curdling produces something inedible, but, when it happens on pizza, the end result is a bit wet, but not completely disgusting, especially if you bake it a bit longer and dry out some of the moisture. It's still something, that you generally want to avoid, though. Packaged fior di latte has the greatest propensity towards curdling with longer bake times, but some of the low moisture brick stuff will curdle as well. Age is one of the biggest culprits. I think it's obvious that you're having issues with the second block because you've had it around too long.
I don't see cheese sales during the summer, but, during the rest of the year, every couple months I'll see brick mozz (polly-o, sorrento, private label, etc.) go on sale for 1.99/lb. I have to be really careful about buying too much in advance, though, because if I let the cheese sit around for more than a few weeks, it will curdle when it comes time to make pizza. Part skim seems to last a bit longer, and some brands seem to hold their own longer as well. Grande never goes on sale, so I tend to buy it when I need it, so I can't attest to how it reacts over time, but I have been amazed at how well it melts. I've seen some brands curdle on me straight from the store, but never Grande. I've also seen some brands fail to bubble and brown on top, but, again, never Grande. Nothing melts like Grande. In a sense, Grande is like the anti-fior di latte. I don't like the lack of creaminess in melted polly-o, but I have to admit that polly-o seems to last longer then most supermarket brands of brick. Sorrento has also given me pretty good stability as well.
When I reach for a non grande cheese, I grab the firmest chunk I can find. Generally speaking, firm (for brick) equals fresh. If, after a few weeks in the fridge, the cheese is noticeably softer, I won't use it for pizza, regardless of whether or not it has spots or smells off.
Soft, white, wet = worst
Firm, off white/translucent, dry = best
Your particular cheese is supposed to be a Grande facsimile, but, from the color and the way it's melted in the past, I'm not really seeing it. The fact that it's curdling on you after less than a month of refrigeration is, imo, the nail on the coffin. Time for a new brand of cheese.