I had thousands prior to my heater blowing crash.
Honestly as Jen stated, I believe it's from over feeding. I did "over feed" intentionally but controled it by extreme filtration. This feeding was for the hundreds of baby Sun Corals I've got. Now they are pretty good size and hopefully capable to survive without continued direct feeding.
These were under a rock, right? They typically live in the crevices of rock, their legs are only seen (come out) at feeding time to grab the food. They don't just lay around on top of rocks as in your picture at least not that I've ever seen!
My thousands were never a problem whatsoever, until the crash! ,
For some reason they were the number one critter that took it the hardest, no other CUC or fish (that I'm aware of) died.
During the die off, it was a major problem and contaminated my tank excessively. I did small, constant daily water changes and was removing large balled up clumps with a turkey baster from the sand bed and wedged in bottom of rocks. There were dozens of quarter sized balls of dead brittles for days.
It's impossible to know if they were a major benefit to my system however, since gone I have had much much higher nitrates. Problem is, I've also added many new fish and reduced the small partial feeding which should have made a major improvement but hasn't.
So Bud, don't know if this has helped you but IMHO they are a positive!
