This is due to the way staff are listed on the system and the way in which Office 365 makes up the login icon using the first character from the first and last name. 

As staff are listed as Mr, Mrs, Ms & Miss Office will take the first character of your name as M and then use the first character from the surname. example Mr A Staff would look like MS for your Office 365 icon.

You can upload a photo to your account to display this and not the name