How nice of you to ask this! Speaking out in a calm voice, but loud enough to be heard , while you are still some distance back is helpful.
I ride in the woods and runners/walkers sometimes see me and then walk off the trail into the bushes , in order to give me more room. However, my horse thinks there is something lurking in the bushes and interprets their move as predatory. If they just speak out, he then knows, Oh , it's only a human being. I swear, sometimes in the shadows he sees movement but doesn't recognize it as a human being.