Rav Moshe’s (EH IV 100.6) defense of the Mishna Berurah (75:2)
(he says that the Mishna Berura is correct) – that below the knee is not ervah; he then says that stockings are required only because of tznius — because if it was actual ervah than transparent stockings would still be prohibited. However neither Rav Moshe nor the Mishna Berura state that there can not be hirhur concerning the lower leg. In fact both state that if there is than it must be covered.
Other poskim assert that there is hirhur and thus the hetair is at most valid in a ideal world which is surely not the present situation.