Is this salt still OK?

[Bartman]

I just went to mix up a batch of new water for a water change using my IO salt and I found that I hadn't put the lid on properly last time I used it. The salt is hard and I have to use a screwdriver to break it up. It seems to (completely) dissolve a little slower than I remember. I'm assuming some moisture has gotten in the bucket. (?) Will this salt still be OK or has some reaction taken place that's going to mess it up? The bucket is still 3/4 full.