Like a slug, but lava. Great game! It feels clean and polished, it feels good to play, the tutorial was clear and levels were well tuned difficulty for a short and sweet experience, and it reminds me of pathfinding algorithms (a bonus) and the classic mobile Flow Free game were you draw lines between points on a grid and they interrupt each other. My first thought was 'Oh no it's the freaking line crossing phone game ahhh'. I could easily see it getting more complicated too. I saw in another comment you scoped it down, I'd say that was a good choice. Something like side objectives would be a good contender for a reward for skilled players. 

Constructive criticism: Feels like a mobile game (probably because I was thinking about the mobile game and the simple design of it). It didn't feel like I was forced to learn anything with how I could mostly complete the levels in one try, reminds me of the GMTK puzzle design video (roll credits) where you misdirect first and therefore learn and analyze the structure that gets in the way. Also, I wish completing the level was more satisfying as it's just a quick cut. Maybe animating the entire room being filled will lava would be satisfying

Yeah going to echo others and mention small tweaks. I wish the green/red blocks were clearly split, sometimes I wasn't sure if it was 4 or 5 total so it made me second guess myself. An undo button with limited uses would be nice (command pattern lets goooo). 

Very nice puzzles, simple start but could become quite complicated levels if they were a little larger or an extra mechanic is added. Would like to see more definition/outline around the goal to make it easier to count at a glance how many squares it's made of rather than comparing it's size to the surrounding level.

Really great idea! Played through the game up to the end, only problems is that not enough levels and lack of Cntl Z 

One of those games that's so simple, but you just want to keep playing!  Good job!

I finished the game! This is cute and interesting and I really liked the music. If you expand this I think it would be cool if there was a button to undo your last trail instead of restarting the whole level.