It's crossocheilus atrilimes not crossocheilus siamensis. True crossocheilus siamensis no longer exist in the hobby. Most common crossocheilus under the trade of siamese algae eater are crossocheilus atrilimes, crossocheilus langei and crossocheilus oblungus.
Other crossocheilus under the trade name of siamese algae eater are crossocheilus nigriloba and crossocheilus citripinnis.
If you want a SAE that eats black beard algae then go for C. Langei. They usually have a much longer pointer snout and thinner body as well as a black mark on the anus area. C. Atrilimes will eat algae but not always as they prefer to eat mosses, soft plants or newly sprouted leaf. Only C. Langei and C. Oblongus are the ones that eats black beard algae but C. Oblungus are hard to find.