That is a fungal infestation called Lawn Rust and is mostly cosmetic.  Lawn Rust are spores that live within the dew on grass that hasn’t had the chance to thoroughly dry.  There is not a treatment for Lawn Rust except to mow consistently and frequently.  The blades will be cut and they will dry up causing the Lawn Rust spores to die.  It is recommended to keep your mower blades sharp and to rinse off your lawn maintenance equipment to prevent spreading.