What is the white on the horses legs in the olympic eventing?

That is eventing grease. Many people used to think that it would allow the horse to slid over the rough wooden jumps on course and prevent injury.It has since been found to cause a decrease in the horses ability to sweat and to actually cause the horse to slip and fall as it melts and slides down the legs to the hooves. It is now slowly falling out of favor with many top eventers.