That's odd, I would expect it to be visible in the "Emission Nebulae" smart album, as NGC 2174. For plate solved images Observatory automatically adds tags for objects in its built-in catalogs that are in the image's FOV. These tags are used by the smart albums. It shows up in the Galaxies smart album because there must be at least one galaxy of the PGC catalog in your image. To make the image show up in the "Emission Nebulae" smart album, add the "en" tag to it. The complete list of tags used by the default smart albums can be found in the User Guide's "Using the Sidebar" article.
