In light of memtest86+ not working with UEFI, is there an open source alternative or something I can use from grub to test memory?
Yes, there is, in the Linux kernel itself: the
memtest option will run a memory check with up to 16 patterns (currently). If you add
memtest to your kernel boot parameters, it will run all tests at boot, and reserve any failing addresses so that they’re not used. If you want fewer tests, you can specify the number of patterns (
memtest=8 for example).
This isn’t as extensive as Memtest86+’s tests, but it still gives pretty good results.