This is an apple tree in western Germany that recently began to show some white fuzz on a couple of snags on the tree. Can you identify this by the shown picture?
Does it any harm, shall I take any measures?
Depends - is there anything resembling this on other parts of the tree, perhaps on the leafed branches? This could be woolly aphid, it could be scale eggs (some scale insects lay their eggs under white cottony threads like this), but if it was either of those, I'd expect to see further evidence on the rest of the tree. Otherwise, it could be some kind of fungal growth, and its presence at a wound site might be troublesome, though it depends how long ago this particular branch was removed. Also, any fungal growths on trees which suggest a serious rotting problem are usually of the bracket kind, totally different from this. Suggest you inspect as much of the tree as you can, including undersides of leaves, to see what's there.