As Dr Joe said is suspended Algae I.e. green water or Algae growth over the tank?
The UV lamp does not prevent algae forming. It works by killing the algae faster than they can make the water go green. The algae will still tend to form so long as the conditions suit them. However if the wattage of the UV clarifier is too low then the water will remain green for the reason that the algae are forming more quickly than they can be killed. If any doubt when choosing an UV clarifier go for a larger size. You cannot overdose with UV!
I have a Uv on my out side pond my tropical pond and my 6 ft tank I have run them for years I think they are a good idea if a tank is in direct sunlight i.e. next to a window they are a must.
Also a UV will certainly kill bacteria and viruses but not as designed into a pond UV Clarifier. Aquarium units are often designed to control bacteria they are then called UV Sterilises, and not UV Clarifiers. There is a big difference.
The CURE for any algae problem is as always; nutrient control. This can be done with regular water changes and limiting light to only what the plants need. Replace 25% of the water every week to remove the abundant fertilizer for the algae. This all depends on tank size and stocking.
