I think this was a visual representation of Eren willing the titan forms metabolic rate into action.
When a body heals, it requires energy, which is diverted from everyday activity (that's why you grow more when you sleep). This, I interpreted as Eren diverting energy from healing towards movement and healing just the bare essentials, like his head.
Of course, since the titans run hot all the time (as in, people who survive being eaten are boiled alive), and he's pushing it to the absolute limits, it makes sense that parts of it would be dumping heat into the surrounding environment. It's sort of like nitro in a car, when there's flame coming out the exhaust pipe.

TL;DR: LOOK AT THE GIANT NAKED MAN ON FIRE

Eren – the first titan super sayan.