What size & shape are the white spots you are seeing... pin head, pencil eraser, or larger? Round or irregular?
It could be algae or perhaps pitting of interior tank metal. Biodiesel will interact & attack various metals. I seem to recall tin lead is on the list among other metals that Biodiesel doesn't play well with.
When I cleaned out my -002a tanks I first soaked with purple power cleaner overnight, rinsed and then ran some other household cleaners. After that I used my 4000 psi pressure washer with one of those rotating tip heads (used for cleaning patio's/decks etc.) inside the tank opening & angled it around. It blasted the inside fairly well. I then poured some acetone in it & shook it around to remove anything missed and also remove any water moisture.
In another one of my fuel tanks I put in some abrasive fill sand and strapped the tank to my rear tractor tire & drove around for 15 - 30 minutes so it would scrub the inside to remove any crud & then used cleaners. Both, methods worked quite well.
I had some other brand tank coating (Red Kote I think) but didn't use it after I read the fine print on it saying; from memory... it was not intended for use in diesel tanks due to difficulty in 100% paraffin removal and the coating could come off & do damage to fuel system components.
However, its now been about 4 yrs and the inside of the tanks need a full scrub treatment again
. I'm thinking this time I going to do the tank coating anyway despite the mfg's CYA warning and just make sure I do a very thorough job.