Slime

A Slime is a gooey creature that first appears in Super Mario World 2: Yoshi's Island and its enhanced port Yoshi's Island: Super Mario Advance 3.

Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
Slimes appear in Tap-Tap The Red Nose's Fort and King Bowser's Castle, carrying keys. They are disguised as rectangular platforms until a Yoshi touches them, which makes them morph into their true form. Like Salvo the Slime (who was actually a Lemon Drop before Kamek's transformation), they cannot directly harm Yoshi and Baby Mario, but they can push them aside.

If a Slime is hit by Watermelon seeds or an egg, it gets progressively smaller until it disappears; it will not spawn any Lemon Drops when damaged, however. The same effect happens to the Slime if it falls into a spike or lava pit.

Yoshi's New Island
In Yoshi's New Island, a single Slime appears in a secret room in Spray Day Mayday. The room can be reached via a door that will spawn by hitting a hidden Winged Cloud above a large block resembling a disguised Slime. The room contains another such block and a few Coins next to it; if Yoshi collects some of these Coins, the block beside will transform into a Slime and start attacking Yoshi as in its original appearances. The Slime can cause Yoshi to lose a life if it pushes him onto the spikes on the left. Defeating the Slime will cause a round of Coins (with several Red Coins under disguise) and a Smiley Flower to appear.

Super Mario World 2: Yoshi's Island

 * Shogakukan guide: ブロックにばけているが、さわるとスライムになる. なぜかカギを持っている. スイカのタネがあれば、カンタンに倒せる. (It is disguised as a block, but when you touch it, it turns into a slime. For some reason, it has a key. If you have watermelon seeds, you can easily defeat it.)

Yoshi's Island: Super Mario Advance 3

 * Shogakukan guide: ブロックに化けているが、さわるとスライムの姿になる. スイカのタネがあれば、簡単に倒せるはずだ. (It is disguised as a block, but when touched, it takes the form of a slime. If you have watermelon seeds, you should be able to defeat it easily.)

Yoshi's New Island

 * Shogakukan guide: 大きなブロックに化けており、ヨッシーが近づくとスライムの形になる. タマゴをぶつけると徐々に小さくなるので、完全に消えるまでタマゴを投げ続けよう. (It is disguised as a large block, and takes the form of a Slime when Yoshi gets close to it. When you hit it with an egg, it gradually becomes smaller, so keep throwing eggs at it until it completely disappears.)

Trivia

 * The Super Mario World 2: Yoshi's Island Player's Guide considers this enemy to be Salvo the Slime. This is changed in the Yoshi's Island: Super Mario Advance 3 Player's Guide.