The coin is a genuine denarius of the Roman emperor Gordian III 238-244 AD, someone had it mounted. If you are very careful you can remove it by using a soft piece of wood to bent up the tabs on the reverse.
Looks authentic and as said, it is a common coin - finding well mounted examples is actually quite rare, it might actually be worth more in the mount than outside it. 'Low value' ancient silver is often soldered in place.
As for 'cleaning' all ancient coins are cleaned to some degree. This one has been 'restored' to blast white, which isn't such a bad thing with ancients and it wouldn't be considered overcleaned.