Prusa MK3 heatbed marks
Prusa ships (or has shipped) with two types of PEI build plates.  The original (and what I was shipped in February 2019) is a PEI sheet held in place with an adhesive.  The marks I've gotten seem to be places where the adhesive has been displaced slightly from long term pressure by the object being printed.

In your case, it actually looks like scratches in the PEI.  If, after cleaning with water, then 95% Isopropyl alcohol, and finally acetone the scratches still appear, I'd check if they are a problem.  Do they transfer onto objects printed on the plate?

If you decide you have a problem, I would use a mild kitchen scrubby sponge to "sand" out the scratches.  I have two types in my kitchen.  The yellow ones are too abrasive.  The blue ones are better.  Gently use one to make the surface uniform.

If you have an adhesive based plate, Prusa sells replacement PEI sheets, with adhesive.  The process of changing them seems onerous.  Although I bought two spare sheets when I got the printer, I have never been tempted to use one.  When my build plate becomes unusable, I will buy another one.

Prusa3D also offers a textured build plate, which is more expensive and in short supply.  These are a powder-coating process and from pictures and their blog, I infer they are particles of PEI which are melted onto the steel surface.  I have no experience with these.

I don't know if they are making the current smooth plates with a powder coating process, or if they are still using the adhesive.

The adhesive is the weak link in the heated bed.  It is only rated for 110 degrees C, which is the temperature limit of the bed.  I find that when I print with a hot bed (such as for PETG), the visible ripples in the surface are worse than when printing PLA on a cooler bed.