This picture is just gross, but the backstory makes it even more disgusting. Apparently, the OP wrapped a plastic bag around a showerhead to collect whatever solids were coming out of it. We imagine they learned of the problem while showing and finding bits of a mysterious white substance all over their bodies. Yuck.
Someone on Reddit pointed out that the substance is not harmless but still needs to be removed. It was limescale buildup, which can occur in pipes, on faucets, and even in bathtubs. There’s a simple fix (a vinegar-water solution), but if the buildup gets this bad, you might need to seek professional help.