Isonychia harperi has been re-classsified as Isonychia bicolor, but that would not be the mayfly you describe, anyway.  It is much too large, and not white.

What I suspect you are seeing is Caenis (click to view).  They are extremely small and white, and have a common name of Angler’s Curse.  They crawl out of the water to emerge, so the dun (or subimago) stage is not generally available to fish.  If you are able to capture one in either of its winged stages (dun or spinner), check it out with a magnifying glass or loupe.  If it has three tails, that’s likely what it is.