citrus trees need help

[Ccat]

My tangerine tree and tangelo tree are in bad shape, the tree trunks and branches are covered in moss and the leaves are covered in a black something, not sure if it's from acid rain, Any suggestions?
thanks

very good response as to cleaning moss off trees, etc.

also, it might be a good time to plant replacement trees in a new, different location.  We sometimes forget that age is not kind to plants, either.  Citrus is slow to mature.

(I adore everything about citrus!)

all the best

Ccat