This will ultimately depend on whether you are a hotel guest or attending an event as a visitor. As a hotel guest, you can wear what you typically would at any other hotel in Ibiza.
So by day, it’s swimwear to soak up the sun, and in the evenings, it’ll turn to casual wear around the property. However, if you are dining at the restaurant, you need to get dressed appropriately as swimwear isn't allowed in the restaurant.
If you are visiting Pikes, smart casual is fine for a club night at Freddie's. However, knowing what is acceptable (and recommended) clothing to wear in an Ibiza nightclub ahead of time will help your packing list.