It can be a tricky thing moving from one island to another in Cambodia. The easiest way to get from Koh Rong/Koh Rong Samloem is to take the ferry back to Sihanoukville and then take a transport by road from there.
note the morning boat will get to Sihanoukville to late for public transportation, so if you choose this transport you will have to leave Koh Rong/Koh Rong Samloem the previous day and stay the night in Sihanoukville.
There are no ferries from Koh rong/Koh Rong samloem to Koh Sdach.
But there are possibilities to hire a private boat. Prices from$150 and up depending on type of boat, weather, passengers, holidays and more.
See the travel information from Sihanoukville