It: the pronoun refers to the mildew (or the “greenish or reddish spot”), and this should probably be made clear in most languages.

It is spreading: the word used here comes from the same root as the verb rendered “to break out” in verses 12, 20, and 25 and may be translated in a similar manner in the receptor language.

You shall burn with fire: compare verse 55. In this case Good News Translation rightly takes the second person singular pronoun to refer to the owner of the object.

Quoted with permission from Péter-Contesse, René and Ellington, John. A Handbook on Leviticus. (UBS Helps for Translators). New York: UBS, 1990. For this and other handbooks for translators see here .