My friend who's using an android tablet game version1.53.0-d20be5c-0 (zz1.54) asked me about this just today - he has this behaviour, asking for playername but if you enter your name in use it says that it's not available. He doesn't want to change his playername so he hasn't tried entering any other name in there. There doesn't seem to be a way to make that "character" go away if you don't want to change the name.
I myself haven't encountered this situation on my ipad (same game version).
EDIT: a cpl of hours later... now I have it on my ipad, german server, game version same as above except ...(de 1.53).
I think there was supposed to be an option for mobile players to change name and this seems to be it. But it should be possible to get rid of this option if you don't want to change name.
EDIT #2: appstore game update description says something like: You can now choose playername if you log in through Facebook.
I don't log in through Facebook, neither does my buddy - we both have created the account with pc, but use those accounts a lot with mobile/tablet.