Hi, welcome to TPT!
Corydoras are not good algae eaters, unfortunately.
But, otos, shrimp, and snails ( Im not sure which snails are best, myself, but Im sure others will pipe in and make recommendations) are good at eating algae.
The problem is each of these algae eaters ( with exception of the snails , maybe?) needs a tank that has grown in a good deal of algae, biofilm, and such to do well.
How long has your tank been up? Cycled?
Do you plan on adding any other fish?
I beg you to NEVER EVER EVER EVER EVER listen to pet store employees and their so-called 'advice'. Common plecos get at least 3-4 feet long. Cory catfish don't eat algae. That is to small for Otocinclus or Corydoras. What fish do you have? What filter? Is it heated? Planted? Shimp or snails would be good. Is the tank cycled? Why do you not want shrimp to multiply? If your fish do well, they will multiply. Breeding is a sign of good health and a happy pet.
Dont forget to send a friendly wave and a hello to the new member!