Tricky, eh? @Jagster is right that having both category and tag icons appearing in posts at the same time will be a bit confusing.
It will be fine if a forum only uses one or the other.
I agree that it would sure be nice if there was a visual way of differentiating tags from categories if icons are used. Personally I’d be happy with also having the tag symbol present for tags and also a category bullet if a category. Or at least have the option to do so.
A tooltip would help (for desktop), but we really need something for both mobile + desktop.