Follow the Treasure Map found in Salty Springs is a Challenge in Fortnite Season 4, Week 3. It’s for finding the treasure of the week.
As usual, there’s no need to find the map. You can go straight to the treasure location.
It’s to be found at the unnamed muddy area between Lonely Lodge and Moisty Mire on the right side of the map. The screenshots below show the location (blue marker on map, click image to enlarge it):
To ensure correct tracking you should play the match until you win or get killed. Then you can quit out by holding / . If you leave the match instantly it will not track this correctly. You can double-check this under the Challenges tab in the lobby to see if it counted correctly.