Can go for months and months with a fairly stable asterina population. Then all of a sudden the population explodes and have them all over tank glass.
They seem to come in waves.
Don't mind a dozen or so working on glass algae, but really looks gross when so many.
Found best way to deal with them is using a vacuum hose to suck up when doing a water change.
Yesterday took out at least 100+ that were on just glass. Lord knows how many still about tank.
Even after those, I can count 30-40 on glass this AM.
After long observation, have determined mine only eat green algae on rocks and glass. They do not seem to eat coraline. So are actually quite benifical so I can live with them in small numbers. Good glass and rock scrubbers.
Literally hundreds upon hundreds in overflows. --- and have recently started tossing captured ones in sump instead of in large planter next to tank whose surface is totally covered with about 1/4 inch of dead ones.
Still, consider myself very lucky that I have the type that are not the more damaging kind.
Expect impossible to ever get rid of all of them, so have just given in to the inevitable and learned to live with them...(at least in relatively low numbers in main display)
Even have a few in Nano that hitchhiked along with LR from main tank.
They are back.... 
