Tip: Layer 3 backgrounds and tides disappear when you hit a message box, so plan your level around that. For example, you can add a sublevel without layer 3 at the start/midpoint, and include your message there.
This is the Lemon Drop from Yoshi's Island, that little yellow slime thing found in one room of 3-4 and used to create the 1-8 boss. It can be made to spawn a single sprite or spawn additional ones like a generator (see the .asm file for more details).
*NOTE: The version that generates additional Lemon Drops may cause lag; it is best used in a place that doesn't have too many other sprites to process. Don't use it anywhere where there could be no valid ceiling for the new slimes to spawn on either, or it will freeze the game.
As of 2017-9-5, this sprite also requires a change in one of PIXI's shared subroutines due to the routine being broken. In GetMap16.asm is the line "LDA #!BankA>>16+1" (it should be line 90), and the LDA should be changed to LDA.b instead.