It’s singular because it’s referring to an alien (one alien) that wants to steal the watch. “Some other” - in this sentence - means “another”. The word (noun) after this (“some other”) can be single or plural depending on the sentence. With “another”, the noun after it has to be singular. If your noun is plural, then you use “other”. Examples: My cat prefers dry food, but some other cats might prefer wet food. She might have passed the test with flying colors, but some other kid failed it. She passed the test with flying colors, but other kids failed it.

In the passage "some other" just means an alien that is not Ben (when he is wearing the watch). There is no requirement that a plural follow the "some other". Perhaps you're seeing the monsters, plural, and thinking that is what the "some other evil alien" was a reference to.

only one alien wanted to get his watch

You can say "Some alien wants" or "Some aliens want". The answer to your question has nothing to do with the word "other".