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Nerite snails, American Flag Fish, Mollies will all eat algae
you can also add some plants that will out compete the algae. Check out the pinned topic on Planting a Brackish Aquarium
I'd say that the Flagfish would be your best bet for hair algaes, and Nerites for slime-like microalgaes. However, not only are both of those animals incompatible with your figure-eight/GSP, but both of your puffer species are also incompatible. The reason is salinity requirements at maturity.
Green Spotted Puffer = Mid/High-End Brackish
Figure Eight Puffer = Low-End Brackish
The only exception would be if your GSP was misidentified as a Ceylon Puffer(aka Green Puffer), which does occassionally happen, which are said to be capable of living in a vast range of salinities. This type of situation is a common novice mistake that will at some point need to be addressed.