Is $300 to treat the underside of the roof and fog a supplemental part of the attic a fair price?

I bought the house a few months ago, and the inspection did not reveal this part of the attic. I'm trying to treat it by spraying antifungal and scraping. The roof is only a year old. There are 10 joists across this part of the roof, with clusters of this substance on the sides of all 10 joists, but only toward the middle where it meets the subroof. It looks like the new roof was put on top of older wood, but the old wood was mostly removed in the same place where this is popping up. The substance comes off pretty easily, is rubbery, bluish, but mostly white/yellow. I was scrubbing this off ahead of new fiberglass getting ready to be blown in. I'm unsure what this stuff is and if it's a major issue. It doesn't look like the dry rot I'm seeing online. The crawlspace under the house looks clean, and right now it's isolated to this sub-attic. I'm also poking around the main narrow attic today. My plan is to chisel these areas off, douse with fungicide, then monitor. This part of the sub attic also has inadequate ventilation and looks like a prior roof leak. I'm wondering if it's on the joists, does this necessitate removing them, or can I douse it with fungicide and monitor? I'm shopping for boric acid right now because RMR141 was suggested but unavailable in my area for a week. I'm using boric acid and a 12% hydrogen peroxide in water solution, and so far it's melting it off pretty good. I'm pump spraying in the attic, using a ventilator, eye protection, long sleeves, and gloves.