The gemara says that R' Yochanan would sit at the gates of the mikva so that the women would see him prior to being with their husbands, which would somehow cause them to have beautiful children who are talmidei chachamim. The Maharsha explains that we see from the story with yakov and the spotted sheep that what one sees prior to engaging in relations has an impact on the offspring. The Rabbonon asked him why he is not concerned about ayin ha'ra. He answered that he comes from the family of yosef so ayin hora won't effect him.
Tosafos asks from a gemara in pesachim that says one who meets a woman on the way up from tevila, if he has relations first "ruach zenunim" will take over him, and if she has relations first it will take over her. Why was R' Yochanan not concerned about this? Tosafos answers either that it only applies to suddenly meeting someone bur R' Yochanan sat their permanently, or it only applies if she has not yet gotten dressed. The Ritva answers that it doesn't apply because R' Yochanan had complete control of his yetzer hora so the woman to him were like "white ducks".
The ritva didn't make this term up. The gemara in Brachos 20a tells of both R' Gidal and R' Yochanan who would sit at the gates of the mikva as women came up. The talmidim asked R' Gidal why he isn't afraid of the yetzer ho'rah, to which he responded that the women in his eyes are like "white ducks". The talmidim asked R' Yochanan why he isn't afraid of ayin hora and he responded as in our gemara that he descends from yosef.
It seems that Tosafos didn't want to answer the "white duck" answer to push off the gemara in pesachim, because the gemara never said that about R' Yochanan. But this really begs the question why did the talmidim not ask R' Yochanan whether he is concerned about yetzer ho'ra and not ask R' Gidal about ayin hora? The answer is pashut. The gemara says that R' Gidal's purpose was to instruct them how to be tovel which implies that he was present at the time of tevila when they are not clothed so their is a concern of yetzer ho'rah, but R' Yochanan's purpose was to expose them to his beauty so he sat further up and only had to be concerned for ayin ho'ra (since the primary purpose was for them to focus on his beauty).
The Rama at the end of y.d. 198 writes that a woman should hide her tevila night and also should not be "pogei'ah" a non-kosher animal or a goy, as the first thing she comes across on the way up from her tevila. The sidrei tahara quotes the sha'arei dura that because of this the minhag is that she is "po'geiah" the mikva lady first. But he has a discussion (based on ma'adanei melech) whether the girsa is "pogei'ah" or "no'geiah" - Pogei'ah would imply that the mikva lady must take a step to greet her, but No'geiah implies that she should touch her. So the ma'danei melech says that she should do both. But, the sha'arei dura writes that R' yochanan's purpose of sitting by the mikva was to make sure that the women were "po'geiah" him first, rather than an non-kosher animal or a goy. From this the sidrei tahara deduces that neither translation is correct because R' Yochanan would not greet a woman, and certainly not touch a woman - therefore the requirement of "po'geiah" is only that the woman who was tovel should see a jew before a goy or non-kosher animal.