why was his face cracking was that supposed to be his veins

It’s figurative- throughout the chapters it is shown Mido (especially his hands) having these cracks. In this chapter it’s alot more apparent IMO because Jichan is breaking away at the walls he’s put up. Mido has become more vulnerable- him telling Jichan he is Mido and Jichan denying it is hurting Mido inside, causing those figurative cracks.