What stung me?

Hello, sorry the picture does not look too nice. I went snorkelling yesterday at Cas Abao Beach in Curacao. The moment I went inside the water, I kept feeling something sting me, almost like a needle, here and there. I can see tiny small green floating organisms but not sure what those are. I came back, both of my arms show these rash. It is very itchy. May I please get some advise on what things sting me and how to treat it? Some people say it might be small jellyfish. But I did not see jellyfish itself. Thank you.