It's a colored and shiny marble, so that it can pass as a lollipop or something like that
no, anon just calls it the "shiny-shiny rock". the fact that he's not showing himself is enough for the yukkuri to pass it as "a talking mister shiny-shiny", and they'll immediately come after it as their new toy.
*P/s: kos shitting and pissing themselves at every single occasion possible (especially the Reimu here) is a normal thing, but I haven't quite seen an adult pissing itself like this.