10 astreas in a 55 gallon tank wont even make a ripple let alone a dent in your algae problems.
YOu should have bought your self 20 astrea snails as a starter crew.
Then a week later around 10 large turbos.
There realy is no rule of thumb as to how many snails per gallon,
some will tell you two some will say 1, some will say 3.
Its all about how fast algae grows form in your tank. Even if you can see the alage it is there. Thats why you see snails grazing on "clean" glass, and on "bare" rocks.
If you have a sand bed or aragonite bed other than crushed coral I would by some cerith or nassarius snails. They are EXCELLENT substrate cleaners.
They keep it turned over and sift if fairly well.
I would think In a 55g reef (from my own experience) you would want around 50-60 snails at least, and around 50-60 blue legged or red legged crabs.
YOu can always look into sea cucumbers to and sand sifting stars but they both tend to ge too large for a reef and knock things over quite freqently.
As well seacucumbers will release a toxic soup if they die in your tank.
Lawmowewr blenny's, tangs, hog fish, are al said to be great algae eaters but its bull shiat. LOL!!!
Blenny's eat the equivelant of maybe a brin eshrimps wieght in algae each day! LOL!! Tang wont touch hair algae in almost all cases either, they preffer MACRO algaes, Hog fish are aptasia eaters but not algae eaters.
You could also look into a pencile sea urchin, they are suposed to be none-coral eaters and I know for a fact an urchin will eat EVERY bit of hair algae you have and then some in a matter of days.