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Hard water mineral build up. Its common. Just take off short head if easy and soak it in a bowl of CLR. If can't remove, just hold bowl up to showerhead until disolves.
It is minerals collecting from the water. Fill a big baggie with white vinegar and tape it on the shower head, leave over night. Scrub with a green abrasive pad in the morning. If that doesn't work try Lime Away.
This website explains it all. I was totally amazed.
The blue-green gunk on your faucet are “lime scale deposits.” That lime scale, or scale deposit, forms because your tap water is “hard” with dissolved minerals. Pure lime scale is white, and you might see it in a teakettle or hot pot. But your lime scale can turn green from copper pipes or fixtures. Removing the scale buildup can help make your faucet shiny again, but only for a little while. If you really want your fixtures to sparkle, you’ll have to fight the problem at its source—which is your tap water.
In the water world, we have different words for talking about the mineral levels in your tap water. Water with lots of dissolved minerals in it is “hard.” But a home softening system can remove those hard minerals to make your water “soft.”
https://indysoftwater.com/2015/11/09/whats-this-green-stuff-on-my-faucet/
I use a product called Lime-away
GET a zipper bag & put some Baking Soda & Vinegar inside. Mix it up & place it over the shower head. Zip tie or twistie tie the bag closed around it & leave it overnight. It should come clean the next morning. Run some water thru it to flush out the holes once the bag is off.