Aqua has polished blue controls and translucency effects, so it's somewhat related to its name. Aero Glass tries to look like glass.
The point is that Marble was something that was first mentioned in a baseless rumor long ago, and everyone now attributes any minor visual changes to being "part of Marble." Probably what happened is that someone saw the Quicktime X interface and assumed the whole operating system was going to look like that, and they invented a name out of thin air to feed the rumor sites.
Marble doesn't exist. It's a fan invention.