Can I call an assistant professor a professor?

My former teacher , who had done her PhD in a top-10 university in the UK in 2009, is an assistant professor in my former school.

On the day when I visited my former school and I saw her at the school canteen unexpectedly. I smiled and nodded to her before I said "Good afternoon, Professor Aiko."

We were talking for a few minutes before she went back to her office.

When I was sitting on a subway train on my way home and I was thinking if it was impolite for me to call her that way.

Can I call an assistant professor a professor, and said "Hello Prof. nickname"?