I just found an unlikely predator of my red bubble algae: asterna starfish. I saw one bowed up on the side of my frogspawn and wrapped around and latched on to what I thought was a new little frogspawn bud. I popped him free -- and it was red bubble algae he was engulfing. Not sure if it means he was really eating it, but that is what I assume by where I found him. I'll keep an eye on the few I have and see where they go. All I can say is their hunger has no chance to keep up with growth, but any help is welcome.