Hello,
From the picture it looks like ecchymosis.Since it is your grandfather who has it,so senile purpura can lead to it. Senile purpura typically affects elderly patients as their dermal tissues atrophy and blood vessels become more fragile. Patients develop persistent dark purple ecchymoses, which are characteristically confined to the extensor surfaces of the hands and forearms.No treatment is required except cosmetic purposes.
Hope it helps.Take care and pls do keep me posted on how your grandfather is doing or in case you have any additional doubts.Kind regards.
Senile purura it is.Not problem,multivitamins help.