It's not nearly as scary or heartstopping as the comments make it sound, but it is imaginative and relatable. Even as an adult I get that creepy feeling like something might slip through a door or window at the last second.
I did get very nervous when the power went out, and when the legs came across the screen. I was expecting a jumpscare. But thankfully, for with heart conditions can rest easy, there are no jumpscares. That's rare these days.
I feel like you could make this longer, with a little more psychological horror. Maybe a sound behind the player, or something that makes them want to look away from the garage door. Maybe the player could approach the door to investigate.
Maybe a small animal could wander by, to give the player a false sense of relief. Like "Oh, it was just a cat".
Overall, I liked it. I also like the design of Mr.Spook.