If the ponies are eating properly and the water conditions are okay, I'd not worry. My ponies change from yellow and white with flecks to grey and black and even show some oranges from time to time. In some instances, it is just a day. Other times, especially when 'courting', they change colors for days. I've seen them change color with change in temperature. I've seen them change color if I'm fussing with the decor. They've changed color when I've introduced a new inhabitant...basically...they do it...because they can.

Again, if they are eating right, and the water is okay, and you see no visible injury or stress, I'd not worry.

I added a small Rio50 powerhead to the bottom, back corner of my system. I have it set on a timer that turns it on for just 10 minutes a day. This stirs up any detritus or leftover foods and sends them into the water column where they can be skimmed out. This keeps cyano and other alga from growing on my sand. Remember, with a shallow sand bed like that, you'll not have much biological activity in there. A deep sand bed, chuck full of bacterium (both aerobic and anaerobic), worms, pods etc. will help keep the system cleaner AND will help with PH buffering. I'm not going to get into a DSB -vs- SSB debate here. I believe in DSB and that's what I use.

The addition of the caulerpa serrulata was a good idea. Not only does it provide additional hitches, but it will help with water quality. Macro algae makes quick use of ammonia in the water. It feeds on everything you DON'T want in there..and provides a 'refugium' of sorts for good stuff. Just make sure you prune it appropriately so it doesn't cause staining or go sexual on you. Granted, there is less a chance of that with the razor type than the bubble type, but it can still happen. You'll notice that the ends will become clear when it is in need of pruning. NOT white, which indicates growth, but clear.

I'd still stick to the recommended flow rates of UP TO 5X the tank volume and no more. Additional flow could stress the ponies. Remember, using sponges on powerheads and such will reduce the flow. A Maxijet 1200 with a sponge on it will NOT put out 100% flow. It may be reduced to 1/2 the expected flow; especially if they are dirty. Finally, the use of sponges in your system could cause 'aerobic bacterium' to develop and raise Nitrates.

Yes, add some blue legged hermits. The scarlets get bigger and sometimes accidentially nip at seahorses (especially if the ponies have alga on them..most, but not all do grow alga on their back sides). This will help with sand stirring and will also help with removal of wasted foods. Cleaner shrimp will also help with this and as an added bonus, when kept in pairs or a trio, will breed and provide snacks for the ponies

Honestly, I think this system looks fine. Watch the sand and algae and all should go well until you set up the other system.

Dave