Erchius Ghost

Erchius Ghost will appear on Moons if you carry any Crystal Erchius Fuel or Liquid Erchius Fuel in your inventory. It will relentlessly chase you. The more fuel you have, the faster the Ghost will be.

If a Ghost is too close to you, you will start losing health and/or energy. This can be deadly. Throwing away fuel won't stop the Ghost.

Countermeasures

 * Simplest way is to run away. Unless you carry thousands of Erchius, a Ghost moves slower than player. As long as you can mine quickly enough (bring a well-upgraded Mining Laser to the Moon), you will be able to mine in relative safety - even with the Ghost still chasing you.
 * You can avoid triggering the Ghost by avoiding Crystal Erchius Fuel and mining Moondust, Moonstone and Moonrock instead. Having these blocks doesn't anger the Ghost, and they can later be sifted or crushed into both Crystal Erchius Fuel and Liquid Erchius Fuel.
 * Equipping Erchius Buddy (craftable EPP) will make you immune: new Erchius Ghosts won't spawn, and if the Ghost already exists, it won't harm you even if you stand near it.
 * Ghost Lantern (a rare flashlight) also protects you from Ghost, but only when you hold it in your hands.
 * Ghost Mushroom Spore (an edible plant) gives you the same immunity for 5 minutes.
 * Ghost Shield (EPP Augment) can be inserted into your EPP to give you immunity.
 * In the later game, you can place the Ghost Shield into your Atmospheric Filter or Atmospheric Regulator to project this immunity across your entire base - which is convenient if your base is on the Moon.

Stronger variants

 * Erchius Ghost (Lightless) - appears on Lightless and Shadow Moon planets. Gets summoned if you carry Effigium Ore. Carrying Erchius will also summon it.
 * Erchius Ghost (Gas Giant) - appears on Gas Giants. Gets summoned if you carry certain ores. This ghost starts damaging you at much longer distance (50 blocks).

Anything that gives immunity to normal Erchius Ghost will also make you immune to these ghosts.

Killing the Erchius Ghost

 * In vanilla Starbound, the Ghost was invulnerable, but in FU you can actually kill it with weapons. However, he drops nothing and will immediately respawn, so there is no reason to do so. (except to delay it if it gets too close)
 * If you acquired some immunity (e.g. ate a Ghost Mushroom Spore) when the ghost already existed, then the ghost won't disappear. However, if you kill the ghost while having an immunity, it won't respawn. It will respawn later if/when you lose the immunity.