That's copepods. Usually they are so small you have to use a magnifying glass to see them.
I copied this article that may help!
Reef Suitability: Mandarin fish are one of the most commonly sold fish, yet they are unfortunately one of the most likely to perish in the average reef tank. The reason for this is their very finicky feeding habits. Many mandarins will only eat live amphipods and copepods (pods) which are found in sufficient numbers only in larger and well established reef tanks. The minimum tank requirement is generally stated as 55 gallon with 50lbs or more of live rock. This is based on feeding requirements and not space requirements as the Mandarin is a very slow, docile fish. Perhaps more important than tank size is how heavily fed and nutrient rich a tank is, since this type of tank will generally support a higher pod population than tanks run under more lean conditions.
Disposition: Very non-aggressive to other fish and other fish seem to pay them no attention. 2 males will fight, so they should only be kept singly or in male/female pairs. Mandarins spend their days carefully checking over the live rock and sand looking for tasty tidbits to eat.
Feeding: As noted above, feeding can be a major issue with Mandarins. Some will take foods such as frozen brine shrimp and bloodworms. Others will refuse to take anything but live foods. Mandarins are extremely slow and hover, much like a hummingbird using their front fins while looking for food. Even Mandarins that take prepared foods have a hard time competing with faster fish for the morsels. One suggestion I have heard that makes good sense, especially in smaller tanks that don't support a large pod population is to build something called a 'pod pile'. This is a few small rocks stacked into a pile into which small pieces of shrimp or similar food can be inserted every couple of days. This pile of rocks provides shelter and a food supply for the pods which allows them to rapidly breed and provide food for the Mandarin.
Hardiness: Very hardy under the right conditions when starting with a healthy specimen which is feeding. They seem to be very resistant to parasitic diseases such as Ick, apparently due to their thick slime coating. Unfortunately, most Mandarins are kept under conditions in pet stores that cause them to slowly starve. Look for sunken bellies before purchasing. Some specimens seem to waste away to the point where they cannot survive, even when put into good conditions. If possible, select a specimen that is eating fozen food.
Temperature: Does well within normal reef tank temperature ranges of at least 76-82°F.
Size: Mandarins can get up to about 3" in length.