You have 3 in a 7 gallon????
My goodness. First SS stars are not scavengers, they are predators. They will destroy sandbeds of fauna extremely quickly, unless they are in a big system with a very deep sand bed. They eat all day long.
I can guarantee you are starving yours. It may not look like it, but you are. It can take many many months, even upto a year before they will starve and die. It will first start to darken, then start dropping limbs to save its core.
Having 1 in a 75 is iffy, let alone 3, and further more 3 in a 7 gallon. There is no food left in that 7 gallon's sand, and there hasnt been for a long time. They need meaty foods, and lots of it, not pellets. Its actually suprising you have gotten yours to eat silversides. Are you sure your other inhabitants are eating it?
anyways, 3 in a 75 gallon is not a good idea. You wont have a living sand bed, and they will all die, a lot sooner then they should.
and no.. they wont help one bit with your algae problem.
they also should not be kept in any kind of coarse substrate like crushed coral. it cuts them up.