Your mystery plant is an Echinodorus bleheri, which is the most readily available Amazon Sword species. Your Anubias and Java Fern should be either tied to a driftwood piece, or some type of rock. This is because if the rhizome is buried the plants will die off, so they're better off when tied to something like driftwood.
Also your "Brown Algae" is actually diatom algae, which is caused by
Lighting: The use of improper bulbs, lack of maintenance, and extended lighting hours are contributors that can lead to all sorts of algae problems.
Solutions: Only use bulbs that are designed for aquarium use, paying close attention to their spectral output; don't bombard the tank with an over abundance of light, and run the lights 8 to 9 hours a day.
Nutrients: Diatoms are most responsive to silca/silicates, but DOCs (Dissolved Organic Compounds), nitrates, and phosphates are food sources as well.
Silicates are most often introduced into aquariums by means of using unfiltered fresh tap water, the wrong kind of sand or substrate material, and through sea salt mixes that contain a higher than normal concentration of this element.
Diatom Algae is very normal for newly set up systems, and tend to go away over time. You can also use Otocinclus Algae Eaters which love diatom algae, and are very efficient in ridding your tank of brown algae.
15g Fluval Flex - Will be a planted shrimp tank