Don't worry, either are right, they are one in the same.
It looks like
Amphipholis squamata an extremely common hitch hiker and considered quite desirable to have. They reproduce will in our systems....you probably have a lot more, they are just not easy to spot....and they only get to be about the size of a nickel or quarter.
Having said that I am not totally positive. The proportions seem slightly off. Eh, regardless, nice beasty
Where is your rock from?