Moon Leech Clot
where it comes from
Nothing spawns these except the Moon Lord. When his mouth opens and the tongue latches onto you, one appears at your feet, so no arena layout keeps them away from you.
what it is doing
It cannot hurt you and it is not trying to. Its only job is getting back to the mouth, and every one that makes it hands a thousand health to whichever part of the boss is still alive. Let a few through over a long fight and the health bar you are chewing on is quietly refilling behind you.
killing it in time
It has almost no health and no defense, so anything in your hands deletes it. Reacting is the hard part, since the trip takes under two seconds and the screen is full of eyes and lasers by then. Learn to answer the tongue animation rather than hunting for the clot. Fighting close to his mouth helps more than it sounds: the trip always takes the same time regardless of distance, so up close they barely move and sit in your fire, while at range they drift lazily and then cover the gap in a blink.