We get that "sand" in the cups that flip upside-down in the top rack (and sometimes the bottom rack). I think it's sediment from the food bits that got pulverized during the wash cycle - normally it rinses away, but in the cups that flip over, the rinse water doesn't drain out completely and a little sediment gathers in the cups.
The only way we've been able to avoid it is to anchor the plastic items so they don't flip over (either pack them tightly or put something heavier on top of them.)
If you're interested in cleaning your dishwasher, though, there are products you can buy to do so. I've seen some in the cleaners section of Home Depot (I'm assuming Menard's, etc., would have them too). You put them in the soap dispenser and run the dishwasher empty once.
My ex-boss once told me that using Tang powder (yep, the orange astronaut drink) would do the same thing, because of the aciditiy.