Fleece is supposed to keep small pests, such as whiteflies, off, but it is easy for them to get in while you are tending the plants, and hard for their various predators to get in to control them. the warmer climate under the fleece also encourages the whiteflies. I would remove the fleece, or hold it up as a canopy, spray with soapy water--as Snoopdog says--every day for several days, and follow up with a good hard spray washing. Garlic spray only repels them before they become established.