What is the name for a marching army?

Perhaps, Army on the move..............

An army actually marching isn't called anything, an army usually consists of more than 50,000 individuals, to get all of these to march together and to find a route long enough to cope with all of the soldiers would be horrendous! The largest amount of soldiers I have ever heard of marching together is a Battalion which consists of less than 1000 men.

If however you are talking about an army en route to a theatre of war, it's never really called an army, it is usually broken down int divisions and such because moving that many men would be a QMs' nightmare!