Does Google Photos allow for "Hidden" albums or photos?
I have seen no indication anywhere, from every setting within the site or app to deep searches, that indicates any such feature exists. The last mention of it I see dates back to Picasa. It seems the feature died there.

I personally have entirely separate Google accounts for different needs, and I keep my photos in those locations that don't belong in my general collection. Not optimal, but the only solution I've found.