Clearly there are some regional differences, but you make it sound like 'deck mud' is some low-grade concrete.......is that right?
You say the mud got wet and turned to sand. If something that just happened to get wet turned to sand, it very likely started as sand. If it was rained on shortly after pouring however, you will often be left with a sandy finish, which often can be dealt with.
If you're in Phoenix, freeze/thaw is not something you need to worry about......crappy construction practices however, are.
Is this relatively sound mud that saw rain right after pouring? You can fix this......
Or did this 'deck mud' turn to sand?
Let's start there..........