It looks like it has some coraline algae growing.
You can't grow coraline algae without an originating source, so I'd personally keep the rock. But if in comparison to the new rock, it looks ugly, I'd ditch it.
It's really up to you.
If you do decide to keep it, make sure you give it a good scrub to get rid of the green algae and other detrius. Watch the parameters closely for the first few weeks as there may be some die-off which could cause a mini cycle (bad if your tank has already cycled, good to kick-start it if it hasn't).
Hope this helps.
- Planted CO2 Community Tank
- Planted Shrimp Tank - in progress