Midorima may act like he isn't a huge fan of the dog, but when he's alone with Merlin he's surprisingly soft with him. Even if Merlin is an outright terror for attention, having flat-out run through the paper sliding door upstairs once because he wanted in while Midorima had been trying to keep him out, well.. it appears that's the type that Midorima tends to get along with despite his objections.

That's why when he opens the door, while Merlin rushes out to greet them, he reaches down and scoops the dog up with one hand before he gets to Takao, and holds him in one arm against his chest to make certain that he doesn't fall from wiggling. And there is a lot of wiggling, because Merlin is greeting Midorima this time since he's familiar with Takao now, which means wiggling and wiggling in his arm to try to lick his chin.

Midorima is tilting his head to the side to avoid it while he responds to Takao.

"Only when the moon is out. I'm normal the rest of the month."