It looks like it was planted too deeply. Have you removed mulch away from the base already? Do you see anything like a root flare or signs of surface roots? Certainly mulch piled up against the trunk can cause bark to split and peel away as well as more severe problems. You always want a clear span of several inches around the trunk before mulch starts.

The moss didn't cause the damage. I don't know what did. The damage is enough to warrant taking it down, before it falls down and does some damage. It has started to heal, but the damage is too great to completely heal over and save the tree. Maybe something was placed between the two main trunks and removed before you bought your house. The damage could also be caused by stress caused by the weight of the snow in winter. Both main trunks lean out. These two guesses are just guesses, but I hope it helps. Mike

That extra smaller branch looks like a trouble maker. I can imagine myself out there with a dremel tool trying to remove it and clean uo any loose bark. Good news is this is a Japanese maple and if it fails they are not typically large enough to create an insurance claim! My bet is the arborists tries to clean up that rotted area and see if the tree has enough vigor to encapsulate it. I think I see the bark regrowing further down in the photo. If he suggests any type of sealer I would be curious as to what it is. My understanding is that stuff is more hopes and dreams than science even if it obviously would benefit from a cure rot cure.

Hi Stephen,well as you've had this plant for 4or5 years now with no further issues,I'd be inclined not to worry too much about it.All looks dry and I can see no worrying dark patches which would need further investigation and you say the plant is otherwise healthy.Though unsightly it appears the plant has got whatever the original problem was under control now.Obviously keep an eye on it but currently it wouldn't concern me.