As per what google say and various answers.
Incubi is a male demon that sit atop a female human in sleep, seduce them and engage them in a sexual act and Succubi is a female demon counterpart of Incubus. In reality there is a set of gender for these demons, but since this is Yaoi, these factors was thrown out in the window.
I made my comment since the title threw me in a confusion(Sorry 'bout that). Uke calls himself an Incubi but he feeds in semen as a nutrient but that's also a job of a Succubi. I believe there's a mistake here. Try reading this Yaoi: https://www.mangago.me/read-manga/baby_sugar_succubus/
Just to clarify, if Mitsu is on the receiving end then he is not an incubi but in actually a succubus. Right?