Small, whitish balls can sometimes appear on otherwise healthy-looking aquarium plants. They are especially common symptoms in tanks where new plants or fish have been recently introduced, or in tanks where water quality is poor. There are several possibilities for what these white balls can be, with slightly different appearances and causes. Determining what the white balls are is the first step to formulating a treatment and possibly preventing the issue from spreading to your fish.
The fungus saprolengia is one of the most common types of fungus found in aquarium settings. It has the appearance of white, cottony balls which can cling to fish, plants and substrate. While it is almost impossible to prevent fungus spores from ever entering the water column, visible outbreaks are almost always caused by poor water quality or injuries to fish and plants. The fungus only attacks dead or decaying plant and animal matter, so appearances are usually limited to plants that are wilting, dying, or otherwise unhealthy. Methylene green or potassium permanganate can be dosed according to package directions to kill the fungus, and water chemistry should be carefully monitored. Ammonia and nitrite spikes are common reasons for saprolengia outbreaks.
Columnaris bacteria is a gram negative aerobic bacteria often confused with fungus because of its similar appearance. Columnaris leads to small, round, white lesions on fish and can form similar growths on plants. The round white balls look cottony, like Sarolengia, but may not respond to the same treatments, and are not always caused by poor water chemistry. Additionally, since Columnaris is an aerobic bacteria, increased water flow and aeration will not slow its progress. Columnaris affects plants where material is already decaying or at lesion sites. Treatment includes removing the damaged plant parts and dosing potassium permanganate or methylene blue.
In addition to possible illness and infection, the white balls on plants may be caused by something much more benign. Many species of fish and snails lay eggs or egg sacs along plant leaves and stems, and a good portion of them are whitish in coloration. Watch your fish carefully for signs of breeding behaviors, and research the species in your tank to determine how they lay eggs and what their eggs look like. You can then opt to either let nature run its course and see if any fry survive, or remove the plants and eggs to a separate fry tank.