The Moon Leech Clot causes a special type of enemy debuff which is taken by the player when the Moon Lord's tongue grabs hold of the player. Moon Leech Clots don't deal any damage, but they inflict the Moon Bitedebuff which prevents lifesteal such as the Spectre Armor and the Vampire Knives. If a Moon Leech Clot manages to reach the Moon Lord's mouth before the Clot is killed, a part of the boss will also be healed by 1000 HP. Since they have no defense, though, they can be destroyed fairly easily with most Hardmode weapons.
The Moon Bite debuff does not completely cancel out the Life Drain's effect, as it increases the player's life regeneration, rather than simply healing the player directly.
Moon Leech Clots will always reach the Moon Lord's mouth in the same amount of time. They travel toward it at a variable rate, remaining completely stationary (or at least apparently so) if the Moon Lord is in its default position behind the player. As a result of this, though it may seem counterintuitive, it is actually beneficial to be closer to the mouth to destroy Moon Leech Clots, as they will not move away as quickly.