The Nemesis expansion featured a mechanic called fading, in which creatures and other permanents would "disappear" after being in play for a few turns. Because the cards had short life spans, their mana costs were lower than similarly-sized non-fading cards.
But how was fading depicted in the card art? Since fading is a "quick and dirty" form of summoning creatures, the creatures had to be shown as unstable--they would be gone in a few turns, after all. So fading cards were shown as "breaking up" or "dissolving" back into the Aether. The creatures in the cards can be seen with oily bubbles dripping upwards off of them.
The first card, Parallax Wave, is a fading enchantment that removes creatures from the game temporarily; it depicts some soldiers and moggs fading away. The second card, Parallax Dementia, is a fading enchant creature that takes the creature with it when it leaves play. The third card is the most well-known fading creature of all, Blastoderm. And the fourth card, Parallax Inhibitor, is an artifact that keeps fading cards around for an extra turn. Its illustration shows a person fading away except for his face, where the Inhibitor is hitting him, keeping him "solid."
Not all fading cards have art that follows these artistic guidelines, but most do. Look at others you have to see if they are "dripping away."