I solved the same task with this:
import scala.annotation.tailrec
def primesBelow(x: Int): List[Int] = {
@tailrec
def iter(primes: List[Int], xs: List[Int]): List[Int] = xs match {
case Nil => primes
case x :: xs => iter(x :: primes, xs.filter(_ % x != 0))
}
iter(Nil, (2 until x).toList).reverse
}