National Geographic has this story on the rise of "Giant, Mucus-Like Sea Blobs on the Rise, Pose Danger":
As sea temperatures have risen in recent decades, enormous sheets of a mucus-like material have begun forming more often, oozing into new regions, and lasting longer, a new Mediterranean Sea study says. [. . .]Until now, the light-brown "mucus" was seen as mostly a nuisance, clogging fishing nets and covering swimmers with a sticky gel—newspapers from the 1800s show beach-goers holding their noses, according to study leader Roberto Danovaro, director of the marine science department at the Polytechnic University of Marche in Italy.
The study referenced above studied these outbreaks over the last 200 years focusing not just on what they contain but looking at frequencies and temperature anomalies as well:
From the PLoS ONE website:
We investigated, by means of molecular techniques, viruses and prokaryotes within the mucilage and in surrounding seawater to examine the potential of mucilage to host new microbial diversity and/or spread marine diseases. We found that marine mucilage contained a large and unexpectedly exclusive microbial biodiversity and hosted pathogenic species that were absent in surrounding seawater. We also investigated the relationship between climate change and the frequency of mucilage in the Mediterranean Sea over the last 200 years and found that the number of mucilage outbreaks increased almost exponentially in the last 20 years. The increasing frequency of mucilage outbreaks is closely associated with the temperature anomalies.emphasis mine
Ok, there's a lot of scientific terms in that description but when they are talking about "microbial diversity and hosted pathogenic species" what is important to know is that this includes bacteria and viruses. Things like E.coli for example.
From Mother Nature Network's site:
Some of them can reach an astounding 124 miles long, forming gigantic gelatinous masses that are not just a nuisance, but a health hazard to humans and marine life as well.
Researchers at the marine science department at the Polytechnic University of Marche in Italy say the blobs harbor dangerous pathogens including potentially deadly E. coli.
I think the study's scientists say it best in their conclusions:
We conclude that the spreading of mucilage in the Mediterranean Sea is linked to climate-driven sea surface warming. The mucilage can act as a controlling factor of microbial diversity across wide oceanic regions and could have the potential to act as a carrier of specific microorganisms, thereby increasing the spread of pathogenic bacteria.
You can read the study in it's entirety at the PLoS ONE website for more detailed scientific analysis. Meanwhile the Nat Geo article ends w/this:
Mucilages aren't a concern for just the Mediterranean, Danovaro added. Recent studies tentatively suggest that mucus may be spreading throughout oceans from the North Sea to Australia, perhaps because of rising temperatures, he said.
"It's a good example [of what will happen if] we don't do something to stop climate warming," Danovaro said. "There are consequences [if] we continue to deny the scientific evidence."
Updated to add:
Where do these blobs come from?
Marine snow (small amorphous aggregates with colloidal properties) is present in all oceans of the world. Surface water warming and the consequent increase of water column stability can favour the coalescence of marine snow into marine mucilage, large marine aggregates representing an ephemeral and extreme habitat. Marine mucilage characterize aquatic systems with altered environmental conditions.
Clusters of mostly microscopic dead and living organic matter, including some life-forms visible to the naked eye—small crustaceans such as shrimp and copepods, for example.
Over time, the snow picks up other tiny hitchhikers, looking for a meal or safety in numbers, and may grow into a mucilage.
The blobs were first identified in 1729 in the Mediterranean, where they're most often seen. The sea's relative stillness and shallowness make the water column more stable, providing ideal conditions for mucilage formation.