Although this phenomenon is abundant on the islands, the glowing plankton can only be seen in places with very little light pollution. As such, if you’re staying on Koh Touch (the main tourist area on Koh Rong Island) or on Saracen Bay (the main tourist area on Koh Rong Samloem), the easiest way for you to see the plankton is by booking a boat tour (you can also walk to Police Beach on non-party nights (15 minutes left from the main pier) and Tree House Bungalows (15 minutes right from the main pier) on Koh Touch, or make your way to Long Set Beach around the corner from Koh Touch).
4K Beach, or Long Set Beach has wonderful stretch of white sand, sparkling turquoise ocean, and tropical jungle wilderness. What’s great about this beach, apart from the stunning scenery and inviting pristine waters, is that it’s a short walk away from the main tourist area on Koh Rong Island (Koh Touch).
The luminous plankton tours cost about $5 and usually depart at 7pm at Long Set Beach.
Sometimes plankton can even be seen on the beach. If you’re lucky, you can see this on some beaches on Koh Rong Samloem and on Koh Rong. Such as Sunset Beach and Lazy Beach on Koh Rong Samloem where there are few accommodations. However, this is only possible when it’s completely dark. That means no lighting from the hotels and restaurants in the area, but also no light from the moon.
According to traveloka.com and hatiensihanoukville.com