There is NO biological distinction whatsoever between "brittlestar" and "serpentstar." None, zip, zero, nada
It is possibly Ophioderma appressum, but hard to know for sure from the photo (could be something else for sure...some ideas but nothing firm). If it is the above then actually will not get all that large and is perhaps one of the best choices for our tanks. Note that many many many of the pictures on the web of this species are misidentifications.