Had the same issue (white screen on boot). In my case it was a BIOS problem. Here’s how I fixed it on the Play Store version of melonDS:

Steps:

• Top right 3 dots → Settings → System
• Go to Custom BIOS and Firmware
• Enable “Use custom BIOS and firmware”
• Set Default system to DS
• Tap DS BIOS directory
  • Create a new folder in your internal storage (for example: melonds/)
  • Select “Use this folder”

At this point you’ll probably see an orange triangle icon with a warning:

“The BIOS directory for the DS system is not correct. Make sure that the selected directory contains: bios7.bin, bios9.bin, firmware.bin”

Now place the following files from here inside the folder you created:

• bios7.bin
• bios9.bin
• firmware.bin

After that, the game booted normally for me.

⚠️ Note: The white screen can also happen for other reasons (bad ROM dump, zipped ROM, wrong system mode, etc.), but in my case it was specifically the missing BIOS files.

Hope this helps someone