It's a parasite. It came in with him. Unfortunately, unless you've quarantined, it's now in your tank. It can kill him if untreated---particularly if he's a tang or clown. Get him out to a quarantine tank and treat with hyposalinity or copper: do not put copper into your display as it will kill your tank and render it useless for all future use. Copper gets into a tank's glue and will not leave.
If he's a rabbit or damsel, you can try feeding garlic laced Formula One and it might go away, but any fish like a clown, tang, angel, rabbit or damsel you bring in is likely to get it from your tank, now, I'm sorry to say.
I got it in mine when a rabbit got stressed out and broke out [he was carrying it in his tissues]. I treated it with garlic and it went away. I have not seen an outbreak now in 8 months. I have good water quality and keep it meticulously. Ich outbreaks are common in new tanks because they're inherently unstable and bad water stresses. During stress, if a fish is a carrier, it will break out. If the water is not immaculate, or if the species is particularly susceptible, it will die. Quarantining specimens before putting them into the tank gives you a much better chance of avoiding this unhappy event.
What to do now? Depends on the species of fish and how bad it is.
