iPhone and Android watch

I have created some watchfaces for my
Android watch on my MacBook with
Facer. The faces are on my iPhone now.
How can I transfer them to my watch?

If they are for Android Wear based smart watch then you need a smartphone with android, or to make sure that your smart watch have Android Wear 2.0.

Next you open facer app on your phone.
If watch face have been approved (it have a “LIVE” next to it in Facer - Thousands of FREE watch faces for Apple Watch, Samsung Gear S3, Huawei Watch, and more ) then look for it in the fscer library.
If not:
Log onto your Facer account on your phone, then go to the website with your unapproved watch face and click “Sync It”.

If you are talking about photo album for Apple watch - I cannot help you - I have never held one in my hand.

Thanks a lot for your helpful answer!
Does it work with an Android tablet
instead of an Android smartphone
as well?

I use android tablet, so yes.