I was in the Cancun area in May (north of Cancun) and was on a beach that had absolutely no seaweed problems at all. It was gorgeous! 30 minutes down the road it was horrible, masses in the water and on the beach, not unlike your photo.
When you go is a factor, exactly where you go is a factor, and weather patterns are a factor, but this year has been particularly bad - I have been in the area 5 times in the last 5 years, and have been pretty lucky, but people there a week before or a week after have had issues. It really is just luck of the draw in that area.