We THINK it's Melica ciliata...and it has produced a few unwanted babies here and there, so it probably is a potential weed. I would never in a million years have planted it on that amazing perch next to the waterfall, but somehow it got there, and for weeks in late spring and early summer it makes a spectacle of itself in this highly visible spot. The spot it's growing in gets very dry and is extremely exposed, so the plant is probably a fraction its usual size: typical Melica ciliata can grow a yard tall if pampered while this barely exceeds a foot. In nature it occurs in much of Europe eastward to Kazakhstan--central and western Eurasia.
Over the years I am constantly surprised at plants that somehow miraculously appear in the perfect spot: often plants I am sure I never planted (sometimes, like this one, I hadn't even heard of them before I had to research them to put a name on them), suddenly appear in just the right spot and proceed to flourish and grow better than dozens, maybe even hundreds of plants I laboriously study, nurture and plant out in just the right spot, only to watch them languish or die. And then a bird plants common Japanese barberry in the Rock Alpine Garden, where it forms a dense mound turning bright scarlet in fall, festooned with red berries all winter, and attracting more attention than all the choice gems in that magnificent garden.
I suppose one can strategically say that we gardeners in the 'Tao' and in the know (so to speak) work hand in glove with nature, collaborating, as 'twere. The results can be delightful indeed!