If it’s spring and the water has just warmed up the first thing that comes to my mind is that your fish have been doing the “wild thing” in your pond. They’ve been spawning. Fish usually prefer to rendezvous at daybreak or at dusk so if the foam sighting is just after these hours that’s a good sign that all is well. Another indicator is if the fish have been wallowing in the plants or if several fish seem to be chasing one. If this is the case the foam you see is actually fish sperm. Lovely mental picture — isn’t it?