The pixie tag is fine as-is. No action needs to be taken.
We do have race tags: see elf, dwarf, tiefling, etc. It's fine to also have a pixie tag.
It should not be a synonym of monster because it is not a monster. It may be ascribed a mechanical treatment of monster-ness in some game releases, but we need to look at this more from a generic point of view.
Note the issue about spell tags is unique to spells themselves and does not inherently set a precedent for any other category of thing. Spells have multiple coinciding qualities that make them uniquely problematic to assign tags to. (Some notable spells like wish are an exception.)