Monroe and Rosalee explain to Nick how they see him when they woge:
Monroe: It's your eyes.
Nick: My eyes?
Rosalee: It's how we know you're a Grimm after we woge.
Monroe: They turn black.
Rosalee: Not exactly black.
Monroe: No, you're right, actually. Black's too weak a word. It's more like infinite darkness. And we see ourselves reflected in that darkness. We see our true Wesen nature.
Rosalee: Took me a while to get used to that.
Nick: Really? You see that in my eyes?
Monroe: Yeah, it's actually very unsettling.
Rosalee: It explains why you get some of the reactions you do. You really didn't know this?
Nick: No, I... well, I guess I should've asked sooner.
Monroe and Rosalee explain to Trubel how they see Grimms when they woge:
Rosalee: So why don't we just start with explaining what a woge is?
Trubel: I know what it is.
Monroe: Okay, well, there's also, like, um, stages. So there's Kehrseiten. And they are non-Wesen, non-Grimm types.
Hank: That would be me.
Monroe: And they can only see us when we want them to.
Rosalee: And Grimms, like you, on the other hand, also can see us when we don't wanna be seen.
Nick: [To Trubel] You ready?
Monroe: Here goes. [He and Rosalee woge]
Monroe: No big deal. [He and Rosalee retract]
Rosalee: See, everything's fine.
Trubel: How do you know I'm a Grimm?
Monroe: Um... It's your eyes.
Rosalee: They get really dark when we woge. It's kind of scary.
Monroe: Yeah, we're not talking kiddy pool dark. We're talking, like, all the way down to your soul, deep-type dark.
Aside from the times Nick manages to corrupt them into what he needs done, Monroe and Rosalee are essentially good people. So why would they be unsettled by looking at a Grimm?
On the other hand, Adalind was evil from the get go. Yet she not only woges at Nick on the street, but appears to do so in defiance.
And what about those Grimms Nick encounters during his investigations, you know, the ones who say, "Ahhhhhhh! You're a Grimm!", then run away?
Finally, there's Juliette. She admits to being afraid to tell Nick she's a hexenbiest, but the only thing she's done is to change into Adalind. At that point has never woged into a crone, so therefore has never seen his eyes. So what is she afraid of? Furthermore, when she gets around to woging into the typical hexenbiest crone, she not only looks directly into Nick's eyes, but holds his gaze long enough to demand a kiss.
Interestingly enough, Nick's reaction is one of repulsion, maybe even a hint of fear? His reaction seems to match what Monroe and Rosalee were talking about.