I don't think it's mold. No sneezing and the coop was built from new materials just two months ago. It has great ventilation and fresh wood shavings are added weekly. Although it's rained a few times recently we don't live in a particularly wet or damp region, we're actually pretty prone to drought.
Will definitely ask about the wormer too. That way if the antibiotics don't help I can try that too. Not a bad idea to have both on hand anyway I suppose.