A friend of mine who propagates corals for income had a Fungia which for some reason started to wane and looked like it was about to die. Was in his display tank. One day the plate looked devoid of life yet there were a dozen little Fungia polyps around the tank. Most made it and grew to be nice looking orange plates. Looks like you might have gotten one really cool hitch-hiker!
Here is an updated photo. It has probably doubled in size and has distinct "tentacles"
It eats brine shrimp and other meaty food when I feed the fish.
It has never moved.
A friend of mine who propagates corals for income had a Fungia which for some reason started to wane and looked like it was about to die. Was in his display tank. One day the plate looked devoid of life yet there were a dozen little Fungia polyps around the tank. Most made it and grew to be nice looking orange plates. Looks like you might have gotten one really cool hitch-hiker!
I think that is exactly what happened to mine. I had a plate coral which had similar coloring. One day it just up and died(or so I thought) and then I find this. Apparently it was budding!