Is cholov akum 100% treif (not kosher)?
If so why does it get a hechcher (kosher supervision stamp)?
And if it's not treif, is it assur (forbidden)?
I think the question confuses three different status of milk
The OU and Israeli Rabbanut (for instance) accept R Moshe's ruling and accept chalav stam in their products, but they mark it so (e.g., chalav nochri in Israel, OU-D in the US which is known to not be chalav Israel).
There are various discussions (out of scope here but see elsewhere on MY) regarding R Moshe's tshuva and whether one should go beyond the strict letter of the law and drink chalav Israel. Also not all poskim concur with R. Moshe’s approach. Some poskim rule that the presence of a live, on-site Jew as the mashgiach is indispensable, and that milk which lacks this supervision is non-kosher (chalav akum).
So to go back to the original question: chalav akum is 100% treif (not kosher), is forbidden and doesn't get a hekhsher. However according to R Feinstein the milk in the traditional supply chain of modern countries doesn't have the status of chalav akum.