Is it possible for hackers to access to my hidden photos in iCloud?

izzat260 wrote:

even if my apple id was compromised, they would still need to use my face id or my password to view the pictures, right?

If you are using iCloud Photos, then your photos are available online and someone can view them with your Apple ID/Password. Another reason why you do not tell anyone your Password and if you have, then you should change it now. Also make sure you are using two-factor authentication, as this will prevent anyone with an unknown device from signing into your iCloud without the verification code that is sent to your Trusted Device.

If someone has physical access to your device, they would need your Passcode to view them.

That's unlikely unless your Apple ID is compromised. Use a strong, unique password and enable two-factor authentication.