Custard splitting ..

I would stir it in too.

The liquid is whey if you give it a good stir it will mix all together again. You get it on some yogurts but just stir and it's fine. Whey is just milk protein.

Just tip it all in a saucepan, stir it and heat. It won't be split once stirred and heated. Even less so once you've eaten it.

I've noticed that custard in a carton seems to separate .
If rushed , I prefer the tinned version , but nothing is as good as the custard that you make yourself !
Either drain off the excess water using a fine sieve or beat it like mad when you reheat it - that means on top of the hob .