Micro Quill is a synthetic material with an oval shape. It is very popular in Central Europe and the Balkans. It is also used under the name Ribbing Quill and, if I am not mistaken, it was made by Peter Durisik. There are several versions, some thicker and others thinner. This is the thinnest version and can also be used for #20 sized flies. I think that #16-20 but also #14 are the most optimal sizes.
Some of the materials are UV reflective, probably the color used for painting confers these qualities.
For example here is a normal nymph made with Mad Rabbit Dubbing in Black Pepper and ribbed with orange. The ribbing is discreet, almost lost in the material. In the light and in the water reflections you can see it the pearly orange, but in the UV spectrum it looks beautiful and intense.
Another one where the head was made with orange ribbing quill. Thorax is made of UV Attractor Dubbing which is UV reflective too.
A simple bug where the ribbing used is orange micro ribbing quill. The fly is tied on hook #16