Ah first time mum, hang in there, a few of us went through the exact same thing around the 7-week mark and it really can be quite worrying! The spit-up mixed with phlegm-looking mucus is actually pretty common at this age because baby's digestive system is still very underdeveloped, so some amount of it is considered normal. Please don't dilute the formula though, that can affect her nutrition and isn't recommended. What helped some mums I know is checking the feeding position (more upright), burping more frequently mid-feed, and making sure baby isn't gulping too fast which causes her to swallow air. That said, if the spit-up is very frequent, the quantity seems a lot, or you notice slow weight gain, it's really worth bringing her to the PD soon to rule out reflux or any formula sensitivity, better to get a professional to take a proper look!