Is there any difference between the grass of American football and the grass of soccer?
Grass on American football fields tends to be very, very short. Football is all about letting fast players play fast. And short grass reduces the occurrence of cleats getting caught up in it, which reduces injuries in a game where sudden changes of momentum resulting from a 250 pound guy running into you at 15 MPH are common.
Soccer fields tend to have longer grass. Probably to slow the ball down and give players a chance to get in front of it, try to minimize how often it goes out of bounds. Much, much less contact between players, so injuries aren't as big of a concern.