most likely it is dust from the rock and sand. You will battle that anytime you stir up the sand for the first few months until you have the biome to support a conch or other creatures that stir up the sand. Just part of the game and nothing to be worried about. To see if it is indeed the sand, just stir up a small spot and see if a lot of dust comes up. I wouldn't go stirring the sand bed up to try and get rid of it though I would just let it be.
As far as adding the extension to the pump, if you have enough flow that won't be necessary. The flow will keep the particles suspended and inevitably they will eventually be sucked up.
How is the stand holding up? That was my initial concern when you first posted
I also highly recommend getting what's called
Live Rock Enhance from Reef Brite. It will do wonders for getting that rock ready to support life. Dry rock is a pain but well worth it to start fresh if you have the patience to let it cook for a while before adding coral. Fish will be fine post cycle but definitely wait for the rock to take hold before tossing corals in. The rely heavily on the bacterial presence. Also getting a few jars of
galaxy pods (4 jars for $139, worth their weight in gold when starting dry)
Also, don't forget to look into the
Seachem Tidal HOB if you want to get some surface skimming action. The 110 is more than a 40B needs but the media chamber for it is massive and you will have enough room to add media in the future like
biomedia (2qt will be more than you need) or
chemipure blue (5.5oz for your tank size) etc.. I have even seen an instance where someone used the 110 as a refugium but you will get mixed reviews on whether a refugium is worth it or not so you will have to do your own research and make that decision yourself. But I 100% recommend the tidal for surface skimming with only HOB equipment. So if you get to wanting that in the future I would look there first.
As far as a HOB skimmer, you probably wont need one for a while. A lot of systems never get one and rely on water changes and physical media to keep the water clear and clean. But if you're wanting one, you can't go wrong with Reef Octopus, but they are pricey and
AquaMaxx makes a great unit that isnt as costly as the Reef Octopus
My first tank back in in the early 2000's was an all HOB tank and I still love seeing HOB setups to this day. You can have a super successful reef with HOB equipment and the maintenance is way easier. Can't wait to watch this tank grow.