That is actually really cool it probably is stressed since its new to the tank.Im guessing it is Calcium Carbonate Sclerites.
The coral maybe stressed or damaged.
There is some information about it here
The Difference Between Hard and Soft Corals It may seem simple enough, one is soft and one is hard, but can you really spot the different between these two distinct groups of coral? The biggest difference, as the name implies, is that hard-coral polyps form hard, calcium carbonate skeletons...reefdivers.io
There will always be small critters like this. This just means that it's an established tank with a good cycle. The fact that its small to be visible on a microscope shows that it shouldn't be harmful yet, if it gets bigger than it can provide an issue.I saw these coming out of my leather this morning they are really hard and break like glass. Any Ideas what this strange thing is?