Yes, it's unabsorbed minoxidil and for me it's a sign that you're over-applying. I used to have a nasty habit of convincing myself that I didn't get a good enough layer on the first time and so I'd drip a little extra onto a few patches, invariably when I woke up the next morning they would be the spots with white, flaky lumps.
Like techprof said: "There is no need to soak your head with minoxidil. getting the thinnest possible coating of minoxidil is the key."
Slarti