i can't see any white on her, but the photo is not clear enough to tell. what i can see however, is the horizontal stress stripe going down the length of her body. that stripe denotes her stress levels are elevated. this could be due to the new additions and she's just figuring out the new situation. did you quarantine the new fish in a separate tank for at least a month or did they go straight into your tank from the store? was the white stuff present on your betta before you added these new fish?
your current water change schedule is quite a bit on the small side, IMO, since less than 20% change weekly isn't enough. i'd bump it up to twice a week, 25% each time, for regular maintanence. but for now, increase your water changes to daily if you can. clean water is the best thing if you have a sick fish but not sure what is going on with it. you don't want to dump medications into the tank if you don't know what is wrong.
there is a chance that this could be columnaris (spelling?).