！
Happy birthday, Aiba-chan!
Literal
Birthday congratulations Aiba-[affectionate-ちゃん]!
お誕生日おめでとう is the standard birthday greeting — お adds politeness to 誕生日. 相葉ちゃん is likely a reference to Aiba Masaki of the idol group Arashi. The ちゃん suffix shows affectionate fandom. A simple, warm greeting.