Mealy tomatoes are often caused by stress to the plant.  This can come in many forms but includes insufficient nutrients, too much water or conditions too hot.  Likely potted plants are in a container with good drainage so probably not water, and this is happening early in the season so heat likely not the cause (unless located where there is reflected heat).  Tomatoes need a balance between nitrogen and potassium and if nitrogen is too high and potassium too low this can cause mealy tomatoes.  You may want to use a fertilizer specifically for tomatoes.