Kiiyan calls Ono Yuuki as “Tomoki” just to tease him.
You see, OnoPin’s personal name, “Yuuki" is spelled as「友 樹」, 「友」alone, can be read as “tomo" as it is the first character for "tomodachi" (友達), which of course, means “friend”, but it can also be read as “yuu”, like in “yuujin”. (友人), which also means “friend”.
「樹」”Ki”, on the other hand is a masculine given name. From it’s Chinese roots, it can mean “tree” or “plant”, or “to set up / be established”.
Kiiyan is just teasing OnoPin by intentionally reading his name wrong, thus giving the air of nonchalance, though, seeing as how completely enamored OnoPin is with Kiiyan, I think OnoPin is just plain happy Kiiyan actually notices him lol.
Well, I hope that answered your question. ^^