Martians are treated as second class citizens on Earth, an elderly man helps out a Martian that was being beaten by children and brings him in. He becomes something of a savior to the Martians; I think they call him "father". I'm pretty sure this was a series of short stories, but I'm not 100% positive.
I suspect the story you are thinking of is Half-Breed by Isaac Asimov.
Jefferson Scanlon, a struggling scientist, is trying, and failing, to develop a cheap and reliable method of generating atomic power. While he is taking a walk to think over his work, he rescues a nineteen-year-old orphan "Tweenie", the off-spring of human and Martian parents, from a gang of teenagers. The Tweenie, Max, had escaped from the orphanage where he was raised following the death of his only friend, a fifteen-year-old Tweenie named Tom. Tweenies are despised and treated as subhuman by the general population, but Scanlon takes pity on Max, and invites him into his home. Max has picked up a scientific education at the orphanage, and within a week his insight helps Scanlon solve his problem and develop a workable atomic power source. Scanlon decides to formally adopt Max as his son.
Not mentioned in the Wiki page, but the orphan Tweenies that Scanlon saves come to call him 'father' as a generic title. You can read the story at the Internet Archive.
I'm pretty sure this was a series of short stories ..
There was a second story called Half-Breeds on Venus.
Half-Breeds on Venus begins shortly after the final events in "Half-Breed". The three Tweenie ships have landed on Venus, and over a thousand Tweenies, led by Max Scanlon, emerge onto an upland plateau. As Max is getting on in years, he gives his elder son Arthur the task of preparing the underground settlement where the Tweenies will live, keeping out of sight of the planet's human settlers.