White stuff in babies mouth first successful breast feed

- I successfully breastfed today for 1 whole hour on the left breast (success)! I am due to feed him again in the next hour and a half, I just noticed whilst he was lay sleeping the had some white stuff in his mouth which must be from breastfeeding as I've only done it today and expressed milk from pump the last two days.. as he is newborn - he was spitting a little back up whilst I watched him but there is a little left in his mouth... I just wanted to double check if this is okay to leave in his mouth whilst he sleeps ? Or do I have to take it out?