The only thing I can think of would be slime particles... was there a "splat" noise coming from above you?

Did it look like Cobblestone with overgrowth in it? If so, that's Moss Stone. That usually means there's a cave nearby. If it was a Slimy Green, then that means it's a Slight chance it could be a Slime. I don't know.

Do you use any texture pack? If you do, then maybe it's something that's changed by the texture pack.

I'm pretty sure it's a slime because they jump around making 'Splat' noises. They spawn only up to 15 blocks higher than bedrock, which explains that you ARE close to bedrock.

You might've found a Slime mobspawner. I havent seen one yet but if you found one you're in luck, since slimes are rare and it's the only thing that drops slime for sticky pistons (unmodded).

I agree.I may not play Minecraft but I know enough to know that it is probably a slime.

I have been thinking of this qustion and it came to mind that the odds are leading to a glitch in the graphics, I have came to this conclustion because I have never seen any other reports on "Slime" coming from the celing of a cave or hole and it is nearly imposable to tell if it is anything more because everyones world is diffrent.