Okay okay okay. I'm about to try and help you out here.
First of all, magic infusion absolutely does benefit from your current int, which is 40. You don't want it higher than 50. With your current stats, Magic +5 gives you more damage than Dark +5 would, I can guarantee that. The only catch is that more enemies resist Magic, but that's why you have an alternate Dark-infused weapon (which is a good idea, nice job. Dark damage is the least resisted type of elemental damage in PvE). Moonlight GS will always be best as a weapon that takes advantage of Int and stacking Magic elemental damage.
Second of all, you have 10 points in Dexterity which are basically superfluous. In fact, to be honest, the 40 points you have in Dex are not worth using the Fume UGS at all. Here is my recommendation:
• Switch the Fume UGS out for a Dark Crypt Blacksword +5; it WRECKS, especially if you thrown on Resonant Weapon, and only requires 10 Dex. You are not a dex build -- 90% of your damage comes from Str, Fth, and Int. So if you use Crypt BS, it renders a whopping 30 points that you've invested in Dex completely unnecessary.
• Use a soul vessel to take those points out of Dex and put them into Int and Fth; since Dark BNS scales with whichever of those two stats is lower, you should use those 30 points to give yourself 50 Int and 40 Fth. The 40 Fth will significantly increasing your dark damage w/ Crypt BS, and the 50 Int will give you as much magic damage as you'll ever need to dish with the Moonlight GS.
• Since you're PvE and therefore don't really need to limit your SL, you could even level further (or SV respec out of some other superfluous stats) and go 50 Int/50 fth, to maximize Dark Damage.
Summary
Yes, I understand, the Dark Crypt BS has lower base dark damage than the Fume UGS. However -- and this is something a lot of PvE-ers tend to miss -- you really want to be looking at how to maximize your total damage output with the soul levels you have, rather than looking for the best base damage weapons divorced from your own stats.
Therefore, if you get rid of all that Dex and put it into Fth and Int as I described, your damage output w/ the Dark Crypt BS will at least equal your Dark Fume UGS, probably surpass it (Dark CBS has S scaling in Dark, while Dark Fume UGS only has a B despite its higher base dmg), and your damage output with your Magic Moonlight GS will increase without any kind of penalty.