The tan or light brown color developing would be diatoms (algae) is my guess. You'll see it on plants and the glass too. It passes with time. Very common with new starts. It's not the easiest with the information available to answer but the grain of the sand looks to be fine. White sand will work but it's hard to keep a clean appearance.
I've read through your recent posts here on TPT in other threads but don't have a clue what is recommended on Dustinsfishtanks, visited that site once for less than 15 minutes and left.
This post will be nothing but questions. What was the actual flood date for the tank? What do know about the water you use for the tank? What dirt, potting mix or soil did you use?
A full tank picture would aid in seeing how many plants you have in place. (Listing them wouldn't hurt)
The first picture has the capping layer looking like about 1/2" at the front glass. Not too thick for the gas exchange but over time you may need more so you can add some maintaining it as the upper layers mixes which is normal. How thick is the soil layer?
Rather than pressing down on all the cap material compressing it I suggest simply looking at the tank. Look at the surface of the substrate for mounding or hills developing where it was flat the day before. Use planting tweezers or a meat fork, chop stick, straight wire, (whatever) just something to poke through the cap allowing the bubbles to release. All this will pass given a chance. I've had a few stems go black and just topped the plants and put them right back in.
Test the water if you haven't (it's not mentioned).
Rather than talk to you about the worry and hype voodoo regarding anaerobic conditions and smells like rotten eggs or hydrogen sulfide bubbles I'll just provide this link.
Two (imo) HUGE considerations doing this.
Remember PLZ that while natural tanks (dirt base) and seeded filters can be stocked from day one go lightly with your first stocking list. Dirt goes through changes going from dry to saturated (submerged) and the rate of break down on the organics changes too. Sometimes it can be more than the tank and fish can handle.
Whether you want to or not test your water. Daily the first couple weeks and for the first two months every couple of days. Be ready to change water if the soil burps ugly readings into the water (it can happen). You might have tanks like mine that run straight through the issues quickly and are trouble free from then on. Lots of plants (including floaters), no hard scape to trap the soil gases, control the light (a big key to dodging algae), watch things and let the tank settle (just a month maybe two). The capping material needs to be small enough to contain the soil yet allow the gas exchange to occur. The sand should work and has for others.
That's the first trade off, attention starting out, more or less high maintenance in the beginning., Things can get busy if a bump in water parameters occurs. All the organic material and the bacteria that chew through it do give you free CO2 for a period of time. Gotta love that.
The second major trade off you make is that rooted plants are there to stay. Removing plants with a good root structure is a HUGE PITA. I had an Amazon Sword that had to go because I placed it wrong when it was small. Cutting around the root ball directly under it I killed the plant taking it out and left all the root runners in place. Thinning a field of crypts means a water change and repairing the cap (adding more cap material). Soil tanks are best as a set it and forget it type of tanking (imo). If you like to change things around, re-scape, swap out plants then NPT is not for you. If you want to top off the tank when the water gets low, trim to make room for the fish to swim and not dose for months it might be what your looking for.