The Post credits scene of Spider-Man: Homecoming depicts Toomes being approached in prison by another prisoner, with a scorpion tattoo on his neck. Given the profile of the actor (Michael Mando) and the tattoo itself, this is plausibly Mac Gargan (The Scorpion).
He appears to have a metallic structure over his prison jumpsuit, attached to his right arm that can be glimpsed briefly as he confronts Toomes.
There is little in universe to explain this yet, but is there anything in the wider Marvel canon that would indicate what this is. It appears to be a brace of some kind, and in order for it not to be confiscated would likely be fixed.