Some people may be unnerved if told another had a dream about them and it is therefore inappropriate to mention it. If it is important that they are notified, better phrasing should be used.
So, apparently this is a thing. I have to say, I'm not too surprised about this one, I mean, I personally wouldn't find it creepy, but it seems to be a very common reaction. What I meant about "better phrasing" is instead of saying "I had a dream about you", it might be preferable to say something like "you were in my dream". The first phrase makes it sound as it the person was the focus of the dream and, even if this may be true, it seems to be better to... avoid mentioning that. The second phrase, however, sounds a little more passive. And I think that's what matters. For a lot of people, directness can be unsettling.