A moss that Beverly Crusher is cultivating when the crew of the Enterprise-D black out in Clues, its growth revealing that a full day has passed.
Diomedian scarlet moss is a plant grown by Doctor Crusher in The Next Generation episode Clues, set in 2367.
The crew of the Enterprise-D apparently lose consciousness for thirty seconds after passing through a wormhole, with only Data unaffected. Crusher had begun incubating samples of the moss shortly beforehand and finds that they show a full day's growth. Along with other oddities, among them the body clocks of the crew, the moss convinces Picard that Data is lying about how much time has gone by.
The truth is that the xenophobic Paxans stunned the crew and Picard himself ordered Data to conceal it. The crew's memories are wiped a second time, and the moss is one of the clues that has to be tidied away.