If jellyfish larva are in the water you may not be able to see them but you can still get stung. Could you see any adult jellyfish in the area, even if they were not near you while snorkeling? Are you sure that it was just seaweed that got blown in and not some groups of jellyfish, too?
Several years ago we were in Roatan and they had a outbreak of jellyfish larva stings. The water was really calm and there were big rafts of small, dark, thimble jellies around but the people getting stung weren't swimming near them. And if any of the adult jellies floated near they didn't bother us - but the larva were bad and we couldn't see them!
Some divers and swimmers had nasty reactions to the stings and got itchy, raised, red welts and looked like they had a bad rash or the measles. We spoke with a German gentleman that had it very bad and he was afraid that immigration would not let him back in the country when they went home because they would think that he was infectious.
The locals called it "Picu Picu" and the divemasters told us to cover all exposed skin before going into the water. They had big jars of Vaseline available on the boats so that we could rub it on our faces, necks, and other exposed areas to reduce the chance of stings. I think that they also had us doing something to secure our suits tightly near our wrists and ankles (rubber bands?) to keep the larvae from getting inside, but I am not sure about that.
They told us that you are more likely to get stung while on the surface, when swimming, snorkeling, or while entering and exiting dives. On the first day, before we realized what was going on, one of the guys on our boat was snorkeling around after the dive and he got stung pretty bad.
And it can happen close to shore. I saw a little boy swimming by the beach suddenly come running out of the water screaming in pain; his mother grabbed him and started rubbing him down but he was really screaming, poor kid.
Folks affected by it said that anti-itch, anti-inflammatory ointments like Cortisone helped to reduce the symptoms. Allegra has a new anti-itch ointment that I've found works really well for mosquito bites, if it is available in your location.
We were lucky, we didn't get any stings on the first day that the jellys showed up, and we were careful to cover up exposed skin during subsequent dives. Right at this moment I can't remember if you are allowed to wear gloves while diving in Roatan - probably not - but I'll bet I was wearing gloves during the Picu Picu dives!
I've read that if the larvae get under your bathing suit, it can really irritate the skin and cause a bad rash that will persist, even after laundering the clothes; but during the Roatan outbreak the divemasters were mostly warning us about exposed skin, so I don't know.
Sea lice can also sting you but I've usually seen them around at night.
After a few days the wind came up and blew the jellies away and there were much fewer problems. Good luck!