Good morning girls, we're 5 months old, we gained only 400g in a month…

Good morning girls, we're 5 months old, we gained only 400g in a month, they said to connect with formula milk, although I was giving 60/90ml in the evening before too sometimes and during the day. After taking it started giving after in a day 4.5 times per 60ml. The baby became calmer... Since birth, I gave NAN Tripple Comfort, but for a month now I give NAN Optipro, seems like the body reacted normally, stool was normal. Today at night two times she had diarrhea with mucus (photo attached), so I think maybe the formula didn't work because she hasn't gained weight and + started pooping this way ?