A growth of sorts in one of my horses eye :/?

A few months ago i bought a Haflinger, and i noticed when i was meeting him for the first time that he had some pink growth in the corner on the inside of his eye. I asked the lady about it and she said he had had it for years and that it was not a problem and did not effect his vision and it was just a "tag" as she put it. I was skeptical at first because i was worried that the eye was going to have to be removed or if he was going blind.. I had my vet look at it, and He said that it didn't seem to be something to be concerned about. After i took him home a few weeks later it seemed to get better. But still if he has had it for years why would it be getting better now all of a sudden? And its not like its contagious to my other horses and it doesn't seem to effect him and neither the lady or the vet seemed concerned so i wasn't to worried at first. But then i talked to the lady again and i dragged it out of her and she said that her vet said that it was probably cancer. I was furious, but my vet was convinced it was nothing to be concerned about and i trust him. But i wonder if anyone has seen or heard of something similar to this and what it is and if there was a kind of treatment that was used for it would be really helpful. And no its not moon blindness.I'm just curious if anyone has seen anything like this and what it was?I've been around and owned horses my entire life, but Ive never seen anything like this. Its quite odd looking and you can only see it when he looks to the side. Thanks!