White, unfortunately is totally useless for IDing as ALL octopuses can be white
. It is their base underlying color and is displayed when no chromatophores are expanded. Some species (and individual animals) do display white more often than others and in the case of briareus (as well as a couple of others), you can usually some of the green fluorescent spots but just mentioning white is not helpful
reddish brown (vs dark chocolate brown or a yellow brown/tan) would more likely indicate O. mercatoris. Spots, as in true round dots vs splotches of white or star bursts are one of the vulgaris patterns. Veining would be another O. vulgaris or O. hummelincki indicator.
Unfortunately, most divers (and pet stores) see octopuses, not a species of octopus and I have little faith in their guess unless they can give a reason for the choice of species and even then it is very difficult to ID one that is this small.
Unfortunately, something this small is going to be a long time in locating. Regardless of species, it will likely be nocturnal for at least several months. Your best shot at seeing it at all will be to set up a red light and leave it on all night (you can also leave it on all day or add a timer but I recommend that the tank never be completely dark). You can experiment with viewing times but I would start at about 11:00 PM and just sit very quietly in the dark in front of the tank and just watch for at least 15 minutes, minimizing movement. If you can locate frozen Cyclop-eeze, I would suggest squirting a little into the tank about 15 minutes before you try to observe. I recommend adding this daily to the tank anyway but if you try it late at night, it may help locate the octo. You can also try placing a tiny bit of finely chopped shrimp on the same piece of LR each night. Once you locate its den, you can try thawed mysid in a long pipette or on a feeding stick (hard to do) and attempt to coax it to take the food by waving it in front of the den. Much patience will be required and continued attempts after failure necessary.
You may be able to create a desirable den. The best luck I have had it to locate a cluster of Giant Purple barnacles (these are often sold in tourist shops but you can usually find them
on-line or in pet stores - be sure they are not coated with a varnish, most are not). You will want very small sizes and maybe 3 or 4 in the cluster. Place a cluster about 1/3 of the way up the water column and stabalize it in the live rock, placing the openings where you can look inside (DON'T use a flashlight). Small conch, cowrie or snail shells (I recommend no more than 2) placed on the bottom substrate will sometimes prove to attractive them. Unfortunately, if you have nice holey live rock, it may opt to live in the internal pockets until it out grows them (Onn is only now seeming to find her first den too small and becoming more visible).
Good luck!
You mentioned "they". Were multiple octopuses caught in the same location? Can you get pictures of any of the others?