Pizza brewer, I have been noticing that one of my favorite brands of cheese has been seeming different lately as well. I have been told a few times that one of the reasons Grande is considered to be the premier brand of mozzarella is because it is always very consistent all year round. I have heard that the cows diet does change from season to season, and that some other brands of mozzarella really show this difference in the finished product. While it seems like there are more issues going on than just this with your cheese, it could be that the combination of the different milk sourcing/diet, and the cheese being green might partially explain the problem. I have had "green" cheese before. It was given to me as a sample from a wholesaler, and he warned me that I should let it age before use. I did notice that the flavor of the cheese seemed to get stronger with a few weeks in the fridge. It was by no means bitter or harsh, though. If anything it was milder, but with a similar flavor.
Could it be that they have repackaged a different brand, but used the same label? Maybe someone accidentally grabbed the wrong loaf for the store to chop up into one pound blocks.