<p>I've got some curious marks on my heatbed.</p>

<p>It appears to be from my black Sunlu PLA+ (I can just feel it if I scrape my finger nail over it) but I can't scrape it off with the metal spatula.</p>

<p>When I try and print over it the filament won't stick.</p>

<p>Any suggestions as to what it is and how you get rid of it?
<a href="https://i.stack.imgur.com/ofS5K.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/ofS5K.jpg" alt="enter image description here"></a></p>

Edit: As Trish noted, apparently Prusa printers don't ship with Buildtak stickers like the other printers I've used, so this is probably a bad idea in your specific case. For others reading this, only do the sanding shenanigans with buildtak or other stickers that you can easily replace and don't mind wearing down over time.
Used to happen to me printing PETG on Buildtak. I ended up using a medium grit sanding sponge to remove the PLA layer. Related, you might want to grab some 1k grit sandpaper for the same reason, it does a great job of freshening up your build surface once builds stop sticking well.