Ive heard Tom say why you shouldnt use salt that has done that as well as why you shouldnt pour water on top of dry salt in a bucket. The reason was the same........but i dont remember....Something about it making the calcium precipitate out or something....????
Technically, you should toss the salt out. When it gets like that, it's pretty much ruined.
If it's a big bucket...and only the top's crusted like that...you MIGHT be able to just toss the clumped salt, and still use the salt below...but that's at your own risk.
In the future, store your salt mix somewhere cool and dry.
When it clumps like this, it's due to moisture. And it's essentially like mixing SW, only with very little water. AFter a couple days, it's gonna get stale, and even ALGAE can start growing in the clumped salt mix. Not only that...the content ratios will be screwed up. Even though it's only a little moisture, things like Calcium/etc can be precipitating in that little bit of clumped mix. Thus when you make your SW, the Ca/Alk/Mg/pH/trace elements/etc can all be out of whack.
Sorry if it's a loss due to wasted salt mix/money...but using it could cost you even MORE money, as the things in your tank get more and more out of whack due to this.