In Bob Marley's song, Get Up, Stand Up, he says:
We know when we understand // Almighty God is a living man
What does this mean? Does it make reference to Haile Selassie I being an incarnation of God in Rastafarianism?
I think you're right; it's almost certainly a reference to Haile Selassie.
Here we see that Marley once said in an interview:
I would say to the people, Be still, and know that His Imperial Majesty, Emperor Haile Selassie of Ethiopia is the Almighty.
He then said:
Wha' dem want? a white god, well God come black. True true.
(Note also that "Get Up, Stand Up" was released in 1973, two years before Selassie's death in 1975.)
And in this interview, Buddy Wailer (the last surviving member of The Wailers) is upset about a documentary because:
Rastafari was what Robert Marley sang about all his life. Rasta music is the legacy he has left us. When I [saw the documentary], I did not see an emphasis on Rasta — our faith, His Imperial Majesty Haile Selassie and the influence these had on the man Robert Marley.
While none of this evidence explicitly confirms my answer, all evidence certainly points that way.
Edit: And note these other lines from "Get Up, Stand Up" (emphasis added):
You see, most people think // Great God will come from the sky
Take away everything // Make everybody feel high
But if you know what life is worth // You would look for yours on earth
And now you see the light // You stand up for your rights
I think that Marley here wants to underline that man is the god - people can be as powerful as a god.