There are thousands of species of brittlestars; I would prefer people not make the kinda blanket statement that they can be problems as the overwhelming majority are not likely at all to be problems (even of the couple of dozen available in the hobby, IMO). The one's that get no bigger than a nickel will not be a terror.
The GREEN is a KNOWN PREDATOR IN THE WILD.
Few others are known to be. Now and then, another causes problems, yes, but I would bet in most cases they are caught with the evidence, and nothing more. They will be cleaners of dead things, no doubt at all.
The "harlequin" serpentstar - such as Ophioderma appressum on this site, would be a nice choice.
Apart from the green, they are, IMO, no greater risk than most crabs (emerald, sally lightfoot) and hermits and bristleworms....which means yes, they are a certain risk but not a great one, IMO.