Yes, iron fertilization of algae blooms risks stimulating harmful red tides, because the science of fertilizing algae blooms is still being developed. No one can say for sure what will happen when algae blooms are fertilized. The ecosystem is very fragile, and little things can upset it very easily. No one can say for sure it won't produce red tides.
Adding unnatural agents to the ecosystem is always a dangerous risk. Although iron fertilization can have some benefits, the unknowns are too dangerous to make this a viable process. Disrupting the natural ecosystem threatens to cause a new outbreak of the red tide organism, which could have serious consquences for the environment.