Elysian park has turned up oodles of old silver coins to the hardcore turf guys in past years. By now, it's been hammered pretty hard for all the classic 4-start deep-turf-silver signals.
As for your particular target: To be honest with you, it looks like just lead slag from some sort of electrical component (ie.: part of an auto battery insides or something). And the appearance of a Jerusalem cross there seems coincidental.
SuperDave, those could be modern (albeit 1, 2, or 3 decades old) graffiti. Yes, even symbols that appear ancient. I remember when I was a kid, during childhood antics, we'd write graffiti on cave walls. Like "Joe was here Jan. 30, 1878". Thinking the next passerby would get excited thinking they were looking at 100 yrs. old graffiti . So too is it possible that symbols you're looking at are nothing more than graffiti. Even old period graffiti might mean nothing, and have nothing to do with any treasure or goodies.