The number of eyes left on the slime has shows how many more hits they'll take to pop!
The other suggestions are great though! I thought about doing that but felt the UI might too obtrusive and I wanted to keep things more minimal - but if I continue to polish more I'd like to do that, especially since internally there is data on the minimum number of moves required to complete a puzzle, so I could even tell you if you solved it as fast as possible for some extra challenge!