Another off-flavor (muddy?) - dry-hopped Pale Ale

We brewed 5 gallons of AHB West Coast Pale Ale as our very first batch. We did the mini-mash and followed the recipe/instructions verbatim. We were blown away and finished off the first batch almost too quickly. It had a terrific clean, fresh taste with a nice color and citrus-ey aroma. Our only suggested improvement was to make it "hoppier".

So, we decided to keep the recipe in our rotation, and brewed a 10-gallon batch of the exact same recipe. The only addition was dry-hopping in the secondary. We split the 10 gallons between 2 fermentors (8 days) and racked into 2 separate carboys (12 days), each with .5 oz of Cascade pellets in bags with glass marbles (all sanitized). We bottled 2 weeks ago.

Last night I popped open a bottle (yes, a bit early, but our first batch tasted great after the first week, if not a little green). It had a beautiful 1" head, lovely color, and pleasant aroma. The first taste was at first pleasant, but finished with a strange flavor I can only describe as "muddy" - like tap water from a drought-area municipality. I also didn't pick up on much more of a hop-flavor than our first batch.

My brew buddy had tried a bottle of his a few nights ago and thought it had a sour tang to the aftertaste.

Now, neither of us is saying this is horrible beer, it's just not near as good as our first batch of the exact recipe, and the aftertaste is disappointing. Our only differences are that we brewed twice as much, and dry-hopped.

So, any ideas? Is 12 days too long to dry-hop pellet hops? Is this type of flavor something that mellows with time? Any input would be great.

Well, I'm still learning as well, but from the info I've gathered, I think 12 days might be too long.
But then again, you are using pellets and I've only researched leaf. I understand that you don't want to exceed 5 or so days using hop flowers as they start to send out "veggie" tastes after this point.

Not exactly the info you were after, but HOP(e) it helps!

It makes sense that the additional step would account for the difference in taste. I searched dry-hopping, and while I saw that some folks leave hops in for as many as 3 weeks (or more), they didn't specify whether or not it was pellet hops. I'm wondering if this whole batch will have the taste or if "time heels all beers" applies to this situation.

Well when I think of "mud", seems to me that this is a prime candidate for letting it sit and age for a while allowing it time to clear.
So, I'm with you and seeing if time will indeed heal it. Worth a try anyway.