Smart Albums in Nitro are visible in Photos, but not in the Photos Smart Album area. In Photos, they appear in the Nitro folder. That’s because Apple does not allow apps to make Photos smart albums. I’m sure that was confusing. I’ll try again: Smart Albums in Nitro are regular Photos albums (like ratings albums), but they are treated specially in Nitro. Smart Albums made in Nitro appear in Photos with a weird series of letters and numbers. That’s expected (and has to do with the silliness I have to go through to make a “smart album” that’s not a true smart album.
You can’t remove an image from a Nitro smart album. You can only delete images from smart albums. This is also true for Photos Smart Albums (try it). That’s because a smart album must provide the correct search results.