What is the difference between a 1928 US 50 dollar bill with a gold seal and one with a green seal?

A gold seal on a bill dated 1928 or earlier indicates it's a gold certificate, a form of paper money that could be exchanged for an equivalent amount of gold metal.

A green seal on any bill dated 1928 or later indicates it's a Federal Reserve Note, a form of money backed not by precious metal but by the credit of the Federal Reserve System.

All modern US paper money is printed as Federal Reserve Notes. Gold certificates were demonetized and removed from circulation in 1933, during the Great Depression.