if there is mold of any kind in the house side with the underwriter and have the mold removed and then the house re-inspected. You are making a sizable investment and you do not want your investment to devalue or cause you health problems. The renters must be made aware of the mold problem or the landlord is liable. Not every house in the U.S. has mold. Mine doesn't, as a first example.
Since this is an FHA loan, the underwriter is bound by FHA/HUD regulations and has no way around the issue except to insure the house is safe to live in. The appraiser did an inspection to determine value, not to look for non-obvious damage. The fact that he noticed the problem during his inspection is an indicator of the seriousness of the problem. The seller is trying to cut corners and will have to disclose the issue to any potential renters if they decide not to sell. You would be better off to find another house if the seller is uncooperative.
Refer to HUD Handbook 4150.2, Valuation Analysis for Home Mortgage Insurance- Revised Sec. (3-6) pg. 3-8, and Appendix D. This is FHA's requirements and can be found online.