monoid vs monad in Scala

Monads are monoids in the category of endofunctors. Therefore, a monad is just one example of monoid, which is a more general concept.

And, though that might be technically true, the most simple answer is that monads and monoids are really nothing like each other, and you shouldn't be trying to learn the difference between them, but just learn them. There's ton of material about it on the internet, easily googled.