You're looking at the results of red tide bioluminescence, a phenomenon produced by single celled microorganisms known as phytoplankton. When washed up by the surf, beaches can look like they are from another world. This neat video shows how they react to touch.
Red tide can sometimes get so dense that they eat up too much oxygen and produce a high toxin density, producing marine life fatalities that includes fish, birds and mammals. Consumption of seafood like oysters and shellfish exposed to red tide toxins can result in serious illness in humans too.
Karenia brevis blooms can potentially cause eye and respiratory irritation (coughing, sneezing, tearing, and itching) to beachgoers, boaters and coastal residents. People with severe or persistent respiratory conditions (such as chronic lung disease or asthma) may experience stronger adverse reactions.