# after the item where there was leprosy was washed

"after they washed the item where there was leprosy" 

# leprosy

Here this probably refers to mildew. See how you translated this word in Leviticus 13:47.

# it is unclean

Something that God has declared to be unfit for people to touch is spoken of as if it were physically unclean.

# You must burn the item

Here "you" does not refer to the priest specifically. It just means someone must burn the object.