Not being a point-and-click adept, it took a few tries to realize the mouse cursor changes over useful items or areas. (The first puzzle is a little misleading since the key is animated, so I thought only animated things were interesting and completely missed the duck in the second puzzle.)
After that, things feel into place more easily. The second-to-last puzzle was nearly incomprehensible visually, since the bits and pieces used to construct the ... (no spoilers) key loosening device ... didn't make much sense in isolation, though I understand the puzzle needed several "parts".
The last puzzle took some effort to discover the "holders" for the blocks. I knew the answer to the puzzle long before clicking enough to find where they went :)
I liked the progression of "feverishness" evolving in the visuals and audio through the levels, and the finale, which served as a good reward!
Some thematic nitpicking: the game says we need to wake up Sleepy Slymon, but he's "clearly" awake during gameplay (until you realize what's going on). Perhaps the player should be "ghostly"/"dreamlike" and the real Slymon be asleep on the bed? This may also help make it clearer what's under/behind Slymon since he's quite a big fellow.
Other than that, the game would benefit from clearer UI about where the interesting things/areas are (either don't make the first puzzle's key animated, or, mention looking for the mouse cursor change in the instructions). I guess using different color or line styles for active areas/items would be giving away the puzzle in most cases. :)