bruising on baby

so as a little back story i have been with my bf for almost 3.5 years. we have a 2 year old son and a 2 month old daughter .. he has always been a good dad and never shown any aggression or violence towards our kids ... now fast forward to 7.25.18 (Wednesday) was my first day back to work and he was alone with both kids for the first time . The following morning ( Thursday) we gave the kids a bath . When he brings our daughter in i notice a big mark on her lower back / butt and i asked him WTF IS THAT??!!!! He said he didn't know and it looked like she had been hit . Now at this point i dont know what to do . I took pics and have been documenting the progess of the spot .... i must add my kids are mixed black and white. my son has darker skin but has white patches on his elbows knees and white spots on his face .... The picture on the left are as of today 8/3 and the pic on the right was the day i noticed it 7/25

I never in a million years imagine my kids father ever laying a hand on them .

is anyone familiar with these Mongolian spots? do they just appear ? i am so sad and hurting and trying to make sense of this 😢😢😢