For the sake of both the plants, and your own health, that will need to be periodically cleaned off. As well as forming an unattractive appearance, fungi and mold will also spread over the walls and through the air onto plants and into your lungs.
You should therefore wipe the surfaces dry with a cloth or sponge every so often. If mildew has built up, it is usually enough to use a commercial spray to get rid of it. However, you may need to work a little harder to get into the cracks with the sponge and spray.
In addition to the walls, floors can also accumulate unwanted compounds. Since many greenhouse foundations and floors are wood, gravel or even just dirt, they will still require some treatment now and again. Wood can accumulate slippery-when-wet mold, gravel allows weeds to grow through, and dirt can become mud. Even cement will need to be swept or cleaned periodically.
If you have a greenhouse roof made of certain plastics, you will discover that over the course of a few years it may well darken or yellow which will ultimately reduce the quantity of light transmitted and also lead to cracking.
Even ordinary glass can yellow over time and become brittle. As it does, it transmits less light and is more likely to break from hail or accidental bumping. Although this will take much longer than most plastics, you should be prepared for the possible replacement cost after seven to ten years or so.
New polycarbonate panels are making their way into greenhouse construction. They transmit light well, have very good heat retention properties, and can last for decades. Nevertheless, the seal between panes can require repairs occasionally. Re-caulking is usually all that is required and can be carried out with a good scraper, a caulking gun and a lot of patience.
Incidentally, condensation within sandwiched polycarbonate panes, and between the panes, walls and other parts can become covered with algae and mold. Be prepared to clean them, though it will not be required as often as glass.
Ice build up, sharp temperature changes, falling branches and other events can require replacing a pane. A modular construction plan can make that easier and less expensive. Plan or select your design accordingly. Of course, remember that there is repair and maintenance of greenhouse watering systems, benches and tabletops as well.