This is a bit tricky because as you say, it could be a potential Mother at this time of year. It is always a bit difficult to answer this sort of question, not knowing all the circumstances. Was there bedding in the box, or was it empty (apart from the hog)? If there was bedding in there, did the hog take it in itself? Was the hog in there during the day or after dark? Did it just go in for a nap, or has it taken up residence, etc.
If it was me, and I was sure the hog was no longer using the box, and the poo was really mouldy and horrid, I would just try to quickly remove that poo. It depends how bad it is. But mainly so that it would be cleaner if anyone else wanted to use it – my boxes have never been used as a permanent residence. But (if you do decide to) open the box very carefully and if there is any sign of babies or that a nest has been made, leave well alone. You are right to be careful, if there might be babies, because hogs have been known to eat their babies if disturbed when they are very tiny and could possibly desert them.
On the other hand, the hog chose to go in there, so obviously didn’t mind the poo. I suppose the hog could have got rid of it her/himself if they didn’t like it there. Imagine it being in the wild. There wouldn’t be someone to clean up after them then. In general, it is recommended to clean out the boxes in the Spring and Autumn – in the wild the hogs probably wouldn’t use the same nest again. Sorry not being a huge help, but you will be best placed to decide what to do, knowing the exact circumstances and the dangers.
It is an endorsement of your box, that a hog has even gone in there so quickly! So pleased for you that you are enjoying having the hog visitors.