In an open cage like that, mold is not a worry. I've never even heard of mold in a solid enclosure, because it's so hard to keep the humidity above 50 to 60 percent, when it's desirable to have 80% relative humidity. IMO use a cool-mist humidifier sold at your local Rite-Aid. Aim the droplet stream at him. Warm-steam humidifiers can burn your iguana unless you get very good ideas about how to keep him from getting in the humidifier's steam output.