I've said it before and I'll say it again...The rice myth kills too many phones!
You are fortunate that your device is not completely disabled but then again, by powering it on, you are accelerating the corrosion (oxydation) process internally. The spot on the display is caused by water. You will have to replace the screen assembly. You should back up your data while you can.
The water is inside the phone, on the logic board and under the shields, even under the IC's. The rice is nowhere near where the water is. So while it may soak up some water vapour, the real problem is the corrosion that is taking place as the water evaporates. The longer you let a phone sit in rice, the more time you are giving corrosion to damage your logic board. The saltier or harder the water is, the more damage will occur.
Here are some things you can do now:
- Open your phone and remove the logic board
- Inspect the logic board, especially around the connectors and look for corrosion.
- Inspect both sides of the board. Unfortunately, 80% of the board is covered in shields. That's usually where the damage is occurring.
- Put your board in a container with >90% isopropyl alcohol and let it sit for a while.
- Use a soft brush, like a toothbrush and lightly brush away any corrosion you see.
- Rinse in alcohol and repeat.
- Let it air dry for a day.
- Re-assemble and hope for the best.
Your best bet is to find a professional repair shop that does water damage repair. They have access to pro-level ultrasonic baths and specialized cleaners as well as the skills to troubleshoot your board.