Husband got weird rash after swimming in smiths reef today

We had a great time today smorkeling at smiths reef but after less than an hour, my husband said he felt like something had stung him in a few spots. We got out of the water shortly after and he had a red rash on several areas of his arms and behind one of his knees. Any idea what it could be? We treated it with a wash and alcohol wipes etc and he's doing better, its much less red but he says it feels like someone burnt those areas with a hot pan from the stove.

Ive read about jellyfish larvae but isnt it off season for those? Any ideas on what else it could be and any tips to treat the area?

I was out there swimming too but had no issues, but he was in a few different areas than me at times.

Regardless of the rash, we still both really loved the time there!