Lucent Imp

Lucent Imp is an enemy in 

Lucent Imps shoot a blue projectile that either homes in towards the player or is aimed towards the giant crystals that reflect the attack into 4 projectiles; the latter do not follow the player. If two of these crystals keep reflecting projectiles to each other, they will stop after a few seconds.

You cannot destroy the projectile with your pickaxe, but it can be destroyed when it collides with a wall or after a period of time. If the Lucent Imp is killed, the projectile stops tracking you and will continue to fly in the direction it was heading until it hits something.