What in the world is this?

[justinl]

oh okay, no doubt, that's a filter feeding sea cucumber. They're basically like any other cucumber except for the fact that they filter feed. Because of that, they don't really care where they sit, sand rock or glass.

Many cukes are toxic so be wary. There is a yellow variant of what you have, same size, that is toxic and may or may not harm a 24g if it dies; they tend to find pumps or just die because they're unhappy. However, i do not know about yours. is it even tinged yellow?