Green alien emoji

[rogueantimatter]

This appeared in a message I got.

What does it mean please?

I think it means it is an emojii that isnt recognised, eg someone used one that you dont have? might be wrong though.

I find it's when people have a certain app. My SIL for instance uses iMessage (iPhone) so when I get smiley's from her (or send them to her) they appear as the alien. When I send or receive them androids I get regular ones.