Where can i find the material data sheet?

The link that surfgirlt · July 14, 2019 left is no longer on the ELEGOO website. If you perform a search on their website for the PDF 2020180831.pdf it can't be found. I did discover a reddit reference to the material though. Here: https://www.reddit.com/r/ElegooMars/comments/cjif9n/elegoo_resin_msds/

When I unscrewed my bottle of resin for the first time, I noticed there was no seal on the opening of the bottle. Should there have been a seal?

I’ve ordered dozens of bottles and there has never been a seal on the bottle opening. As long as the plastic bag the entire bottle comes in isn’t tampered with/ripped open it should be fine.

Can this be used in an oyumaru mold and cured with a UV lamp?

It won't work well for casting. I tried doing something similar (with the translucent version); the cured resin does not transmit UV well which means that only a thin layer (a few mm) on the surface will cure. Also, it gets fairly hot when curing so it might melt the plastic you're using for the mold.