I had an amphipod explosion one year into my tank. When they run out of food they start eating things in your tank. Mine ate up my poor sea cucumber. They would crawl all over him at night and he would wrinkle up where they were biting him.
I bought a psychedelic mandarin and the problem went away in a few weeks. They cant fit the mature amphipods in their mouths, but they eat the babies, which eventually brings the population back under control.
A word of warning. Mandarins need a lot of established live rock or they will starve. I would say a 55 gallon tank heavily stocked with live rock and really light on the fish load is absolute minimum.
BTW nothing will mess with your mandarin. They are poisonous. However, male mandarins will kill other males of the same species.