There is a celebrity and I like I daydream about him sometimes. It's a celebrity so I never expect to ever have or be with this person in real life.
Is looking at or thinking about good-looking celebrities zina of the mind/eyes?
Abu Huraira reported Allah's Messenger (ﷺ) as saying. Allah fixed the very portion of adultery which a man will indulge in. There would be no escape from it. The adultery of the eye is the lustful look and the adultery of the ears is listening to voluptuous (song or talk) and the adultery of the tongue is licentious speech and the adultery of the hand is the lustful grip (embrace) and the adultery of the feet is to walk (to the place) where he intends to commit adultery and the heart yearns and desires which he may or may not put into effect.
The type of zina would be the adultery of the heart and the zina of the eyes. About the eyes, don't look at photos of anyone from the opposite gender out of lust let alone 'good-looking celebrities'. That part is non-negotiable. As for your heart, try your best to control your desires, seek forgiveness whenever you feel yourself thinking what you shouldn't be, and don't do it knowingly.
And those who, when they commit an indecent act or wrong themselves, remember Allah, and plead [Allah’s] forgiveness for their sins —and who forgives sins except Allah?— and who do not persist in what they have committed while they know. Their reward is forgiveness from their Lord, and gardens with streams running in them, to remain in them [forever]. How excellent is the reward of the workers! (3:135-136)
InshaAllah, you will be forgiven.