I have a Celebi on my Diamond game which was received from a transfer online. It came with a blank expression emoji. Looking back at it now I cant see and other Pokemon with this marker. Any insights to this Marker is much appreciated.
That mark means that it had Pokérus and was cured. Pokérus is a virus that Pokemon can get (like the flu) that multiples the EVs they obtain from battling Pokemon by 2.
Pokerus is a very useful and VERY rare virus in the Pokemon world. It's rare to get it in the wild, but it's easy to come by now that online trading is so easily accessible. This virus will allow Pokemon to gain Effort Values twice as fast as usual.
Source Hope I helped!
