Fog Crawlers are not your only issue in Fallout 4's latest DLC, Far Harbor. Many PlayStation 4 players have had problems playing the DLC because of the FPS drops. Recently, Bethesda has deployed a fix that changed the FPS number back to 30 FPS. After the initial release of Far Harbor, foggy areas of the island caused a severe drop in the performance. The players had their FPS dropped down to 20 and sometimes even to 15.
Now everything's fine. If you are still having FPS drops on your PlayStation, simply deleting the add-on and re-downloading it again should fix the problem. Here's a comparison video. The amount of fog is reduced, and the quality is better.