I'm not sure why people are certain that it's Moria who has this ability. I certainly wouldn't say it isn't a strong possibility, because it is, but you could easily make a case that it's not him.
Take for exampleā¦..unless he's an amazing incredible fighter of his own ability, taking away a shadow in their current location means jack squat.
Obviously whatever it is that took Brooke and the old man's shadow (who is possibly the guy from Brooke's flashback?) still retains them, but what does that mean/what advantage does that create? They're just shadows, does controlling them prove any tactical battle advantage? I mean, if you could control shade, that's a pretty lame ability by itself. Now, if you go into talking about controlling darkness itself, that's something else BUT we already have one of them.
So the question is, what battle advantage would taking shadows as a latent DF ability prove on this island shrowded in darkness?
Edit:
Oh, and um, about people already complaining about Gecko being there when all we know about him is his name.....
-_-
In addition to what Ivotas already pointed out, let's not forget Hogback already showed interest in them because of their bounty. Whether this relates to cash for something like research or the more likely idea....SPARE PARTS....his position here is remarkably strategic with respect to exactly what kind of pirates would be passing through.
Not to mention it's even easier to justify the location, being close to Marine HQ and all, given Moria's protected status.
Just be patient. I know it's been a long time since everything has been spelled out for fans and maybe this is the first time for some of you to be in this stage of the comic if you joined after/during W7 and before EL but just because everything doesn't instantly make sense, don't discredit it.