You're not just seeing things after visiting the Medical Marijuana shop. There really is a face up there in the black paint.
M&F has not been to this spot so we cannot vouch as to the potency of the meds and whether they make you see things. Stoned or not, what happened is Snyder used to have his Rocket Boy riding on this building. When he wall got a new paint job, the painters painted over everything except for the face of Rocket Boy.