That would be sludgy urine, not the poop. Too much calcium can also tax the kidneys. But a little bit excess calcium in the diet isn't usually a problem for most rabbits, and it will just be excreted in the urine. It's only an issue if it's causing thickened urine and/or calcium build up in the bladder.
It just depends on the crop that the hay came from, whether or not you might find some alfalfa in the grass hay. Usually no, you won't find any or maybe just a tiny bit. Unless the farmer is specifically growing a grass alfalfa hay mix, which is a common thing for some horse hay bales that are sold. Crops are growing plants, so you will sometimes get a batch of grass hay with a few weeds, or maybe that little bit of alfalfa. As long as there isn't a lot, it won't normally be a problem. If there is a lot of alfalfa in it, then you can always exchange it, and/or contact the company for help with the issue.