He looks Greek in that photo, but I don’t think he was.
Some folks make a career out of character acting.
They give up having their name at the top of the marque, in exchange for getting more work and more exposure.
If he worked with director John Huston (Angelica Huston’s father), he must have been impressive early on.
He was of southern Italian heritage, and like many of us, probably has Greek ancestry way back.
IMDb says Forster has English, Irish and Italian heritage.
The Huston film you referenced, “Reflections in a Golden Eye” was a strange one. He initially had it photographed to make it look like the film stock was soaked in a vat of urine, which didn’t go over too well (I think the studio had it corrected to normal). Forster’s first film roll was just a minor one, but he played a soldier that Marlon Brando was obsessed with (Brando playing a closet-job Army officer). Elizabeth Taylor did a good job playing Brando’s “grotesque” wife, one that topped even her shrewish role in “Who’s Afraid of Virginia Woolf.”
Francis Coppola used a lot of photographs/stills from this movie of Brando for the dossier of Col. Kurtz in “Apocalypse Now.”