Is it white, like the scales are white? Almost looking like theyre grey/silvery?
Is the whiteness fuzzy at all? Stringy?
Does it look at all like this?
Even if darker, more white/silver, this isnt anything to worry about. This is natural coloration.
If fuzzy, stringy, then please let us know.
Your tank isnt in too rough of shape. The temp is alright, you use conditioner, you feed it well, and the water changes arent too bad.
Sometimes, bettas need a little while to settle in. They may not eat for a bit, which is sometimes common. Has it ever* eaten while youve had it?
Keep offering the food. If it doesnt eat in 5 minutes, remove the food. Consider another type of pellet, maybe smaller ones that fit in its mouth better. Avoid "freeze-dried" anything.
The breathing heavy may or may not be a bad sign. I have had two healthy, happy bettas that constantly breathe heavily. I dont know why, but it doesnt seem to be indicating issues. There is an issue when it is breathing heavy and also gasping at the surface for air or coming up very, very frequently.
Are you acclimating the fish properly? This may be why he is stressed out.
Do this for us:
Right now, or when you can, do that 100% water change, complete with a rinse of everything (no soap! soap will kill the fish), and water conditioner. Make sure, using the thermometer, your temp range is from 77-82'.
Keep the betta in his cup with the old water from the tank. Once youre done with the water change, allow the cup to "bob" in the water for about 5 minutes. Doing this will allow the fish to get used to the new temperature of the water. After 5 minutes of bobbing, add in a little bit (like a few spoonfuls/tbs) of new tank water every 5 minutes for about 10-15minutes.
You should acclimate this way each time you do a water change, even in a 50% change. The acclimation will get the fish used to the new temp and water parameters, which will help it relax and avoid shock. This is important.
Can you attach pictures, btw? It will be very helpful.