RE: Pattern on bottom of prints
Can't tell from the photo what you consider "pattern" and what's reflections. In any case, I'd take the sheet to the sink and give it a good scrub with plenty of hot water and fragrant-free dish washing soap. That'll take care of any crud on the sheet that may lead to imprints. And if you use your sheet often enough, you may end up with some patterns even the sink treatment can't handle, in which case I switch to a new sheet. They are consumables even though they last longer than one might think.
Formerly known on this forum as @fuchsr -- until all hell broke loose with the forum software...
RE: Pattern on bottom of prints
That first layer isn't quite low enough too as there are gaps between the filament lines. Its also a bit uneven, are you using 7x7 levelling or 3x3 ? Ideally you want to switch to 7x7 if you haven't already.