Moop stands for Matter Out Of Place and is pretty much a catch all term for anything that was not on the playa before we arrived. Since all that was there before was playa dust and compacted playa dust, essentially moop is everything that is not playa dust.
Most commonly it is used to describe litter and since Burning Man is a leave no trace event - there is no amount of moop that is acceptable. Causing moop is very bad and threatens the event's lease renewal.
Moopy is a term commonly applied to clothing or other objects that shed pieces easily. Garments with sequins, feathers, glitter, beads, rhinestones, and similar items tend to be moopy. Although they look nice, avoid feathers - no matter how well put together you think they are. Other objects that you think might shed some moop should be tested. One popular testing method is to slam it against a table a dozen times or so and see if anything at all comes off.
Do Your Part
As citizens of Black Rock City it is our civic duty to pick up moop wherever we see it and educate those you see dropping moop about why it's wrong. Your camp should have a leave no trace attitude and a plan to go with it.
Never Let It Hit The Ground
The number one thing you can do to prevent moop is never let anything hit the ground. There is almost always a strong wind and chances are anything you drop will be moving very fast in an eye blink. For the same reason never leave anything that might blow away unsecured - even for a moment.
Cleaning up moop is called Mooping.
The Burning Man Earth Guardians are a prime source for leave no trace info. You can also volunteer for a shift with them. Click here For More Info.