The tag wiki for algorithm consists of a summary:
This challenge is intended to be solved by using, creating, or resolving some processing algorithm.
and a detailed explanation:
This challenge involves algorithms.
At first glance this seems to be about as useful as tagging questions programming. One can just about make a case that questions which can be answered by a program consisting of a single literal don't deserve the tag, but we have more than enough of those questions already.
Does algorithm serve any purpose?