Crystallized old pee?

Let me start out by saying that this could be from a cat, I don't know -- I'm watching two dogs and three cats right now. Everyone is healthy except one of the dogs, who is not only old, but has an infection going on through her whole body right now. The vets think that she's rejecting her artificial joint. Oh, and she's on antibiotics for it now.

Anyway, I came across some old urine. and it smelled so off that I'm not sure who it came from, the dogs or the cats. It also might have been from as long as a few days ago, but no more than that, and I think it was from this morning (I woke up a little late to take them out).

So, when I found it just now, it almost had this slightly crystallized nature to it. When I wiped It up with paper towels, it was incredibly dark.

Should I be worried? Or maybe its just a cat thing... Her current health problems are what's making me a little concerned.

Thanks!