I am star crossing one Ezreal Prodigy mine, and opponent's Rengar, when the effect is on the chain I want to heedless the Ezreal and get a Fizz, can the Fizz use that star cross?

To answer your question: No, your new Fizz, Trickster cannot use the Star-Crossed that is currently on the chain.

Here is the breakdown of the timing and interaction:

1. The Interaction

• The Chain: When you play Heedless Resurrection to kill your Ezreal, Prodigy and play Fizz, Trickster, these actions resolve before the original Star-Crossed.
• Targeting Loss: Once Ezreal is killed by Heedless Resurrection, he moves from the battlefield to the trash. Per Rule 359.3.e.4, because the unit has changed zones to a non-board zone, it is no longer the same game object. The targeting relationship between the original Star-Crossed and your Ezreal is severed.
• Fizz's Ability: When you play Fizz, Trickster, his "When you play me" ability triggers. This ability instructs you to "play a spell from your trash." Because Star-Crossed is currently on the chain, it is a Finalized Chain Item (not a card in your trash). Therefore, Star-Crossed is not a legal target for Fizz's ability.

2. Resolution Sequence

1. Original Star-Crossed (A, B): Placed on the chain targeting your Ezreal and your opponent's Rengar.
2. Your Reaction (Heedless Resurrection): You pay the cost (kill Ezreal) and place Heedless Resurrection on the chain.
3. Heedless Resolution:
   • Ezreal is killed and moves to your trash.
   • Fizz, Trickster is played to a battlefield.
4. Fizz's Triggered Ability: Fizz's "When you play me" ability is added to the chain on top of the original Star-Crossed.
5. Fizz's Ability Resolution: You must select a spell currently in your trash to play. Star-Crossed is still on the chain (a spell is not in the trash until it has finished resolving and moves there per Rule 156), so it cannot be selected.
6. Original Star-Crossed Resolution: It attempts to resolve. Since Ezreal is no longer on the board, that target is invalid. Per Rule 359.3.e.8, the spell will resolve using only the remaining valid target (Rengar), returning Rengar to his owner's hand, while ignoring the instruction to return Ezreal.

In summary, Star-Crossed is not in the trash when Fizz triggers, and even if it were, the targeting relationship with your Ezreal was severed the moment he was moved to the trash by Heedless Resurrection.