You aren't supposed to be out here, little dude!
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The Silver Harp can only have its effect happen on the overworld, so playing it in caves does nothing... that said, the map has plenty of places you could play the harp on, and it can result in quite a few oddities...
We're fighting the basic Slime in Goldman territory - that doesn't happen naturally.
The chances of you ever summoning a Goldman (or other strong enemies to grind) with the Silver Harp are extremely low, if not outright impossible (at least in this version of the game - this might have changed in remakes), so the Silver Harp is basically useless outside of being a silly trinket. It IS a quest item, though, so we eventually gotta hand it over to the NPC that asked for it, otherwise we're not really getting anywhere.