Something is very off about this being, and everyone knows it.
Except it's not. When someone is very off, people steer clear. The creepy guy who hangs out in front of the supermarket makes his creepiness known by asking out any woman unfortunate enough to engage him in conversation for 5 seconds. The creepy little girl likes to talk in depth about dead animals she "happened" to find.
People talk and joke about creepy people they know. They do this to confirm their beliefs and to warn others. It's a form of social bonding, in a way that protects them from the weirdo...it can also be a form of discrimination.
Because there is sometimes that fine line between marking someone as creepy and discrimination, or just plain unfairness, people are reluctant to do it in borderline cases. They're more likely to label (either out loud or to themselves) when it's a stranger they won't see again, but even then, they may just shake it off.
What you want is a set of very subtle differences your character exudes. Any one of them can be passed off as misspeaking, being distracted, or just a personality quirk. It's the totality that give people pause.
Because of that, there is nothing specific anyone can point to. They won't mull over any one incident, because it doesn't amount to much. If you want all this out of not just your character's words, but also out of their thoughts, it has to be a pattern only the subconscious picks up on.
Have you ever been around someone and you suddenly start thinking about movies you saw, TV shows, books, etc? You might not even realize the person reminds you of someone else, just that hmmm, oh, this just popped into my head.
What if most every time one of your characters interacted with your eldritch, they turn to their companion and say something like, "Let's see a movie tonight! Oh, I know, how about Us?" Or they start talking about the real life haunted house in their town growing up. Or maybe they and their companion each get a quick chill.
Any one time, it doesn't mean anything. But the reader will see this over and over and get a clue, even if the characters only understand this on a subconscious level.
So what are the little things the eldritch can do that feel "off"?
I'd start with things that aren't actions or words.
People give off energy and an eldritch would have a very different energy. This would be another reason why a character wouldn't label the person as creepy or weird. Most people aren't willing to tell someone else that person's aura is off, or whatever. Unless they're already super into New Agey stuff (this is old agey stuff, but not in mainstream culture).
- Get a chill when passing the eldritch.
- When in a conversation or other interaction, your emotions feel like they're wrapped in cotton wool. You're not depressed, just muted.
- The outside world is slightly muted as well. You don't pay attention to anyone else until someone resorts to tapping your shoulder or calling your name.
- There's a pit in your stomach. Hunger? Ate something bad? Drank cold water too fast?
- You realize your child is clinging to your arm so hard you almost drop your bag.
- Your legs feel heavy, rooted, you want to end the interaction but your body isn't listening.
Then there are things that the eldritch actually does or says.
Little things they get wrong, as if they aren't from around here. There's no accent, no verbal tics, but something is not quite right. Can't put a finger on it.
- Speaks to the same to children and adults. I don't mean avoiding baby talk, but more that they don't understand that people of different ages comprehend things on different levels.
- Language is slightly too formal for the occasion.
- Aside from a couple pat phrases, a complete inability to engage in small talk. With just enough self-awareness to make people think it's a quirk, not an inability.
- No fidgeting. Nothing. No touching things to feel their texture (clothes, hair, plants). No playing with their own fingers.
- No swaying or adjusting weight (some people move because it feels right and others need to adjust weight to avoid soreness). They're not at military readiness or anything, just still.
- A second longer than average to respond to questions/conversation.
- Complete (but brief) answers to questions, no more, no less.
- Nothing personal, ever. No "my husband and I love that restaurant" or "time to go pick up the kids."
- Clothes, makeup, hair, shoes, etc are exactly so, but not looking like a professional do-over or anything. They don't look like a model or actor, but there's nothing out of place either. No wrinkles or scuffs or faded lipstick. Hair looks real and not sprayed in place, but it's not frizzy or poofy or in an off position.