Hi there.
That is just a type of Chiton.
More specifically, a Cryptoplax spp., probably a Cryptoplax striata, or Cryptoplax larvaeformis.
What gives this away is that most chitons have 8 fairly noticeable shell plates. However, in the Cryptoplax family, most tend to hide/reduce the plate size by hiding them in the skin. Notice that in your photos, there are 3 plates which are visible. The rest are just "the tops" of the other plates that are hidden in the skin.
Harmless in your tank, and not dangerous to humans. They eat algaes. When I ordered live rock from Tonga a couple years ago, there were about 10 of these same exact chitons dead inside the box.
Best,
Ilham