Based on the first picture I was thinking a chiton of some sort. I've seen some tiny one's in one of my tanks that I don't know where they came from. But picture #2 seems to show a small hole in the shell that indicates limpet of some sort. I have plenty of limpets in my systems, they just show up, but they're usually super white. If it's hard shell that come to a point it's probably a limpet.
You know what? I'm not going to be wishy-washy - it's a limpet!