Looks just like the crap I'd get on my rims and fenders after a trip to the town dump--we used to have to drive to the top of a "mountain" of trash,garbage & mud,and a huge loader/dozer with steel tires would push the refuse over and cover it with that mixture of gravel ,sand and something resembling cement..
Once it dried ,which took only minutes,it was rock hard and you had to use a hammer to get it off..if you left it on your fenders and rockers it would eat holes in them,like it was laced with acid!..
I had to remove my tires and use a wire brush on a drill to get that crap off,then I waxed the hell out of the rims with Butchers paste wax..that helped to keep any more from sticking for awhile..
Mud can suck to get off--the '77 GMC I had was "mudded" by a previous owner--I tried pressure washers,car washers with hot high pressure soap,nothing I tried would take all of it off..I drove it almost 12 years and it had just as much mud sticking to the cab,frame and undersides as day one..
I hear linseed oil works to keep mud off or let it wash off easier,but it has to be applied before any mud gets on whatever you want to protect..