Høvel is made of Brass and comes highly polished out of the box. Brass is a reactive metal that will naturally tarnish in different conditions and especially when handled. Oils and moisture from hands will affect the brass colour and will tarnish it helping to build a patina. This can be easily cleaned off if you prefer it looking perfectly clean and polished.
The first step is to take the Høvel apart, separating the screw and clamp from the main body. Use some Brasso wadding (or other brass cleaning agent), this can be purchased at most hardware stores, supermarkets, and online. Take a small piece of the webbing and in small circular motions wiping the tarnished areas and sections of graphite. The wadding should start to turn a deep black showing that it is removing the oils and dirt.
Once you have cleaned the brass take a soft cloth or tissue and slightly moisten it with some water and washing up liquid/soap. Then wipe off the residue from the Brasso in a similar motion and care to the Brasso wadding. Then run the parts under some warm water tap and clean off all the cleaning residue. Finally taking another soft cloth/tissue and dry any moisture, I usually use a yellow glass polishing cloth because it will buff up the Høvel to make it very shiny.