If I purchased a home under a USDA loan years ago and mold is covering the attic, under the home and potentially is in the walls, what do I do?
You can remediate the mold--have it professionally eliminated, etc.--but you'll have to pay for cost for doing so yourself unless:
1) The seller gave you some sort of home warranty or guaranty: if they did, you can enforce it as per its plain terms, and it may entitle you to compensation.
2) You can show that the mold was there when you bought the house, and the seller knew of the loan and failed to disclose it to you, and the mold was not something you could have reasonably discovered yourself (or with a home inspector) prior to buying the house (that is, it was not in some accessible place you, as a buyer, could have seen), and you bought the house less than three years ago (statute of limitations). If all those criteria are met, you may have a case for fraud against the seller, to sue him or her for compensation, but you need all those criteria to be met.
Otherwise, while unfortunately, this is simply one of those expenses that a homeowner must sometimes bear.