I don't know where it came from, maybe from me, but it's nothing more than a light olive thread body with a cream hackle tail and cream hackle collar in size 18. It's just a general mayfly leaning toward a PMD. It's always been a pretty good producer for me for hatchery fish.
The BS fish have always seemed susceptible to size 18 dries that resemble patterns, but don't copy. A bastard Adams, with nothing more than some brown and grizzly bookending a thread body can also work, but not as well for me as the Celery. EHC or G gnat. I think size is more important than color at times because I believe there are a number fish who will "taste" anything that looks buggy.