Sounds like its definitely string algae then. One option that would not only curb your string algae problem, but would also prevent problems with Herons would be to drape shade cloth over your pond this Spring. Spring is when problems from these two pests are most common.
Shade cloth is something both of you should consider. You can buy it for around 75 cents per sq. ft. right now.
Owner, Par 5 Lawn Care