쌩(Sseng) is a strong versions of 생(seng). 생 means 'raw.'
얼(eol) is the first letter from 얼굴(face).
Many Korean celebrities post their 쌩얼 pictures so that they show off their face without any make-up.
But the funny thing is most their 쌩얼 face is not just make-up free. They do their 쌩얼 make-up by applying some BB cream and very light lip gloss and etc. etc.
So... if someone says her face is 쌩얼 today, don't believe her 100% because she might have done some 쌩얼 makeup. :P