Yep, looks like a bacteria bloom from your tank still settling in. Going to happen from time to time. "something" happened to throw off your water chemistry briefly. Carbon will do nothing to help with this and the amount of carbon in those filter pads is all but useless.
You're running into one of the headaches of having a 30 gal nano-cube. They can be tough to keep balanced ........ but yours is still relatively new as well..... so it's going through normal growing pains. Try adding some of the "bacteria starter" to see if you can help get your good bacteria growing on the rocks a little faster..... or you may not have enough rock all together for the amount you are feeding.... ect. ect. ect.
If your ammonia, phosphate and nitrite testing is reading ok I would not do water changes until needed. Give it a couple of weeks. Keep testing the water daily to make sure no nutrient spikes are happening.