Depends.
If any self labeling is valid then yes...
If you go for the no god definition, if the entity is not a divine being, then demon counts as divine but ghosts or souls in general not really they would be more part of how universe works in the "non objective reality part".
But lets say the no spiritual or supernatural based ideologies, then no, believing in anything like this would make you part of some kind of religion (even if it is only you) and thus not an atheist.
But words and fables are not a fix thing, but when using the labels to discuss the concepts you need to be precise otherwise you can be talking about a rationalist objective form of atheism and suddenly someone says that "Buddhists are also atheists" aaaaand the conversation is gone because now you are discussing the definition of a word

Might depend on how high they are. Maybe 'meds' are off or need adjusting. Some common over the counter drugs can cause adverse reactions. Lidocaine makes me see things especially if I'm drinking Tequila. I have several friends who have "Spiritual" experiences and see dead people when they trip. I'm not judging. Some of them think it's real. Perception is reality to most people.

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the·ism
/ˈTHēˌizəm/
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noun
belief in the existence of a god or gods, especially belief in one god as creator of the universe, intervening in it and sustaining a personal relation to his creatures.
"there are many different forms of theism.

[google.com]

Less broadly, atheism is a rejection of the belief that any deities exist. In an even narrower sense, atheism is specifically the position that there are no deities. Atheism is contrasted with theism, which, in its most general form, is the belief that at least one deity exists.

Atheism - Wikipedia [google.com]

Atheism addresses the question of god/gods. Ghost are not deities. Some may or not believe that demons magical creatures or gods. Many atheist, barbers, golfer's and even Libertarians, may believe many things.

I don't think so. If they call themselves 'Atheist', they shouldn't believe in ghosts. Simple.

Clearly so. Phenomena have nothing to do with atheism unless you put them into a theist context

You've answered your own question in the first sentence...

Not belief but curiosity at some unexplained phenomena

Belief is too strong a word, but I was told a very convincing ghost story by reliable people. It’s very mysterious.

I’ve had experiences that I cannot explain. I lived in a house with my kids and my first, then my second husband. All of us, and several visitors, saw this...well, we just called it ‘the lady’. I’ve had an experience of being out of my body. As an RN in Hospice, there were some incredible things that happened.
I’m agnostic; I’ve seen no proof of any type of god. I can’t explain the other things...it’s a mystery.

Why not?