On the brick wall in the background, you can see the remnants of an old painted ad: the words "Meet me at". And below that is the rounded top of something red, which looks to me like it was part of the original ad, but could also belong to the motley collection of graffiti and graffiti-covering paint that now obscures the rest of the ad (zoom in).
Here's one theory suggested by a simple internet search: The wall was painted by a department store to advertise its in-store Santa Claus. As was the case with these examples, "Meet me at" would have been followed by the name of the store, and the rounded red thing would have been the top of Santa's hat. But would a store really have paid to paint a wall with an ad for a short-lived seasonal attraction? All of the examples I linked to above were little buttons; that seems like a more sensible medium for advertising your Santa Claus.
In case you're wondering, it seems likely that the ad was once fully visible to people here on Liberty Avenue: I'm not sure what year the ad was painted, but a 1951 aerial image shows that the supermarket building in the foreground had not yet been built at that point. The ad would also have been seen by people passing by on the elevated rail line overhead (today's A train), as well as those waiting for the train at the eastern end of the 88th Street station.