I actually can't seem to see the pics, but there are two common hitch hiker brittlestars.
A small white one with 5 arms - Amphipholis sp. This is a self fertilizing simultaneous hermaphrodite that broods its young. It reproduces quickly, stays smaller than a nickel or so, and is highly desirable as a general sand bed etc scavenger.
A small brittlestar, often banded, with 6 arms - and Ophiactid brittlestar that reproduces by splitting. Stays small, and also considered highly desirable.
Not all green brittlestars, FWIW, will be predatory. They are KNOWN predators in the wild, so it is unfortunate that they are imported, however, they are also unfairly accused of a lot of things.