'Glacier Mice' were first described (and named) by Icelandic researcher Jón Eyþórsson in a 1951 letter to the Journal of Glaciology.
They are found on the surface of glaciers, and are typically 7 to 10cm in diameter, being composed of small stones covered in multiple species of moss, and sometimes hosting other species, such as nematode worms, springtails, and water bears.
Their mode of formation is unknown. They have been observed to move, very slowly, across the glacier surface in a non-random (sometimes grouped) way - which is also unexplained.
Further reading (with photos) in a 2020 article for NPR.