If I get you right, he wants you not to use the term "I" but rather "your girl" when you write something here or adresses Him in rl right?
If so then I guess it is a control issue. Good or bad but I think that is what he means.
It is not that uncommon in a Master/slave relationship but for me I dont really feel that micromanaging what my sub calls herself is something I care about.
If he wants you to but you dont then take it up for discussion. It might mean more to you than it does to him or the other way around.
Hard to know since I dont know either you nor your Master/Dom but those are my thoughts nyways.