close

I think it's one of those tags that could go either way. I would consider the context of the entire post. If it were subtle or accidental I'd feel comfortable calling it questionable. If it were blatant, obvious or on-purpose I'd say explicit.

If I could see areola from the thumbnail would say explicit but I don't feel strongly enough to change it. There is usually something else (nopan or erect nipples) that will push it into explicit.

I noticed however there are quite a few that were tagged as safe.

https://chan.sankakucomplex.com/?tags=areola_slip+rating%3As&commit=Search

Unless they were accidentally mis-tagged, I think aerola_slip should at least be questionable.