Yeah a filter sock is just fine in a sump for carbon... supposedly it doesn't even need brisk flow over it as the media itself attracts contaminants. GFO is probably less effective in a sock however.
It is probably better though to use less carbon and change more frequently than to put a huge bag of it in and let it sit for long periods of time. After a while it can develop a bacterial slime coat and be less effective.
I use a TLF reactor for carbon and GFO these days (mixed together.. not layered.. as it will clump that way). It has been very effective. Before that I did use a filter sock. I went with the reactor mainly for the gfo. I change out about every 4 weeks.. though I have lapsed for up to 6 weeks.
I rinse my carbon and GFO very well, but even then for the first day after a change the tank water takes on a bit of a color. I think it is just something that isn't easily avoided. It clears up soon enough. Settles, gets skimmed out.. etc.
I wash my carbon with cold tap water in the kitchen sink (this is ok!). I put it in a bucket.. fill up with water. Swish it around with my hand. Pour most of it out.. repeat. If you want stick some RO in there and let it soak for a while, but I don't usually do that. After a good rinse and the water runs clear to mostly clear I do a final drain and it goes into the tank (reactor or sock.. depending on case).