This type of can is called a coated can, which is shinier and more luxurious than ordinary steel cans.
The clumps can easily be undone to a fine, beautiful powder using a sifter. You can sift Matcha ahead of time and store it.
Matcha tends to clump up due to static electricity, and clumps sometimes remain on the tongue if used as is. Therefore, sifting the matcha before whisking makes it finer and tastier.