The bubbles could be kinda hard to pop at times. You can also pop them as they spawn to prevent them from spreading entirely, you could prevent this by making the bubbles invulnerable for a small amount when spawned. I think the main thing missing here is an object, maybe having to protect people from the fog bubbles could interesting, something like the book/move The Mist. Or if you were going for a relaxing vibe it could use some nice music, sound effects, and a simple high score system.