Azolla typically comes in via a duck or other water fowl when the plant gets stuck on their body. From there, the weed multiplies faster than rabbits! It took two weeks for that pond to get covered which is lightning quick.
The best way of managing it initially is chemically. The best product to use would be Sonar followed by Reward. From there, nutrient management is key. That can be accomplished by adding aerators under the surface to get pond circulation. The problem for us is that we don't have power out that way to do that. We are looking in to having Clean Lakes do a Sonar application, but it is very pricy. For now, we'll have to enjoy the nice contrast between green and Cinnabar Red.