Howdy! I've collected and kept a number of briareus, and the second picture in particular does indeed look very similar, with that distinctive mottled green-and-pink coloration. I would agree that the webbing doesn't look quite as prominent as you sometimes see, but they can expand and contract it to a great extent, no? So while I can't say for sure, I wouldn't rule it out, either.
If it is a briareus, they're fun guys, and pretty hardy. Remember to keep a tight lid on the tank, of course--I had one that kept escaping despite my best efforts--gave my mom quite a shock one morning when she found him staring up at her from under the dining room table! He was fine, but he eventually got too big for the tank, and had to take him back to the sea.
Good luck on keeping him!