Someone could have given him spoiled milk and Rav Moshe did not want to embarasses the person for doing so , so said it was Chalav Stam as they would not be as bad as giving him spoiled milk.
But how would Rav Moshe tz”l know that it was chalav stam without asking the person?
I think you totally missed my point. I dont know if the story is true or not, I am suggesting that they story might be true but for a different reason and IMO my version actually shows greater midot of Rav Moshe.
I really dont belive that Rav Moshe vomited Chalav Stam , because it was Chalav Stam and people are using this as a reason not to use it. It would show that Rav Moshe was a Hippocrite (CHV) as he said one thing but did another, However if he was accidently given spoiled milk and did not want to embarass the giver, This shows his mida of not embarrasing someone who did an accident and one should not yell at someone who Puts too much salt in the soup , Closes door on your fingers or accidently gives you spoiled milk.