There is a lot of problem algae going on there in those photos. Fortunately all is due to the same thing, an imbalance of the light and nutrients. When algae increases to the extent it becomes a problem (nuisance, killing plants, etc) it is always due to this one factor, an imbalance.
Light drives photosynthesis, and each plant species has a minimum intensity requirement; intensiity is one aspect, another is spectrum, and the third is duration. An insufficiency is any one of these cannot be corrected by increasing the others, and vice versa. The intensity/spectrum is likely not the issue here in the sense of being inadequate, but the intensity may be too strong. A 20g long tank is shallow, so more light is reaching down into the water than would be the case with higher tanks (which is why they generally require brighter lighting) so here reducing the duration might help; you haven't indicated the duration, so it is difficult to suggest changes that might help. Floating plants also help this a lot.
Nutrients are the second aspect of the balance. Plants can only use the light fully for photosynthesis if all 17 required nutrients are present. As soon as one is insufficient, photosynthesis will slow, and may stop altogether. In such cases, algae is quick to take advantage because it can use any light and fewer or imbalanced nutrients. I have had problem algae increase from too much light (either the intensity or the duration), and from too little (intensity), from too many nutrients and from insufficient. Fish food and water changes provide nutrients, and obviously any plant additives.
Knowing the daily duration of the light and if any fertilizers are being added will help us.