Likely an isopod, serious but need not be the end of the world.
Take immediate action to remove it with tweezers, dab the wound with iodine. Make sure it doesn't fall back into the tank, you will need to capture the host fish; best to use a clear bag or acrylic trap for that, then with a plastic baggie on your hand hold the fish gently but firmly (avoid touching the fish with your skin) so it doesn't flip around. Plan it out first, and hope their aren't others. Keep an eye out for more in the future, if you see several more you will need to treat with copper or if you have inverts let the tank run fallow without host fish for a month or more.
Move all wounded host fish into your quarantine system if possible.