Rav Moshe, when writing in Hebrew called Reform and Conservative clergymen “rabbis”, spelling out the English term in Hebrew letters in middle of his Hebrew Psak. This is, in fact, Rav Moshe mocking them. It is similar to putting quotation marks around the term “rabbi”. And he didn’t use the term for individual Reform and Conservative clergymen but rather only when discussing them as a group.
Like in his Psak saying it is assur to enter a Reform or Conservative house of worship sanctuary.