That is red slime algae. It's most commonly seen if you are using tap water, are overstocked or are overfeeding. Overstocking is a problem in your tank. The trigger needs a 180 gallon tank:
Those fish are way too large and eat way too much for a 55 gallon tank. Your water quality may be suffering from the inordinate amount of waste they produce and the amount of food you are required to feed them.
Red slime algae is also commonly seen in tanks that don't have enough flow. Do you have enough powerheads?