leaves on oak dying and falling off

[hedgesparrow]

have some small oaks in a cemetery i look after one is in a not good way. the leaves are curling up then turning brown and dropping off had a look and found a aphid like creature but it was silvery grey with 3 legs or antenna out front no bigger than a match head. i see the others have got it but to a lesser extent. any ideas what it is and will it harm the tree.

if i can will try and download photos of it if that helps