Well, I think it's the washer fluid itself... it's all sitting there in the tubes ready to go, but in the sun it heats up and expands and some dribbles out of the nozzle onto the bonnet.
I think if you left it it might affect the paint, so best clean it up as soon as you can.
If you search on the internet it seems to have been a problem with previus minis too and some people have had a fix applied that involved fitting some extra check valves into the tubing. My dealer knows it happened to my car but I said no hurry to fix it (now winter is here). I'll see what they suggest when the car next goes in.