I agree with Opaeforms, they look like amphipods of some sort. Even though Hyalella azteca can live in brackish conditions, I do not think they are Hyalella azteca specifically because of how long the front antennae and arms are and because they came with chaetomorpha, suggesting they are probably marine amphipods. A lot of marine aquarists grow chaeto in 'refugiums', a type of planted tank connected to the main display tank. These serve two main purposes: nutrient export and a sanctuary for zooplankton (amphipods, copepods, ostracods, mysis shrimp) that make their way into the main tank over time, providing fish with a natural source of food. Sometimes seahorses are kept in these refugiums, as they will only feed on live food and enjoy a more macroalgae-heavy environment rather than a traditional coral reef. If these are marine amphipods, you may be able to kill them off by reducing the SG.
Nevertheless, the behaviour of marine amphipods will probably be similar to H. azteca. I know H. azteca hunts smaller freshwater crustaceans, they maul and devour Daphnia. Most amphipods will eat anything - plant matter, animal matter, dead or alive, even each other. I definitely think you made the right choice by keeping them away from the opae ula!
Last edited by Halocaridina on 29 Mar 2017 15:30, edited 4 times in total.