Halo: The Mona Lisa is in my opinion, the best look that we have gotten on the flood excluding the books. It lets you see how horrible this parasite can be and also how vulnerable we are to it, this also helps out when making us feel a sense of danger throughout the story.
Sadly, you can tell that this animated movie lacked of a big budget, and most, if not all of the characters felt underdeveloped, but this isn't a massive problem as the style that the animation goes with allows the low budget. Even though the characters aren't deep in their personalities and are mostly one sided, you can totally understand their behavior and they all fall perfectly into the situation they're in.
I would have totally loved to see more developed characters and a bigger budget for a better experience, but the sense of danger and the other feelings the movie brings to you, just makes the movie a Halo worthy experience.