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There are 3 things algae needs to survive, water, light and food. Make sure that fertilizer is not making its way into the reservoir, as this can make algae blooms worse. Also, if you can, block light from getting into the reservoir. Third, make the water less friendly with a few drops of bleach. Literally just a few, 1-2 drops per gallon of water and don't add it right before you plan on using the water in the reservoir. This will be diluted enough not to harm your plants but will reduce the algae and making sure to wait before using the water adds an extra layer of protection as the bleach will evaporate from the water, leaving even less in the water.

If you want the look of the natural wood, then you could go with a clear sealant (which will help guard against moisture from humid conditions inside greenhouse). Most people go with the latex paint option, however, adding an oil-based, bare wood primer with stain-blocker - followed by "exterior" latex enamel. I have also heard (though unsure of its effectiveness) that adding a few drop of vanilla extract in the paint will help inhibit mildew. You could always ask about this whenever you get your paint. The main thing is to go with something that will not be affected by the humidity, which could warp the walls.

I would check greenhouse suppliers.

You also can use plastic to tent the plants.
You also need to provide air circulation as to not cause mold and fungus growth.

There are many different ways to shade a greenhouse. Attaching the shading will depend greatly on the material and construction of the greenhouse itself.
If the walls are plastic you can use adhesives to attach some type of hook system to the walls. You may have to do some exploring at a home improvement or hardware store.

Many greenhouses bought from kits will have methods to attach shade cloth.

I personally drape landscape burlap over the inside rafters of my greenhouse. This filters the sunlight just enough in the early summer sunshine.

If you have a wood structure you can attach hooks to loop the shade cloth through.
Remember that that shade cloth will only be needed when the sun is so strong as to burn the plants.

https://www.gardeningknowhow.com/special/greenhouses/greenhouse-gardening.htm

Here are some articles that have information on growing these plants.

https://www.gardeningknowhow.com/ornamental/trees/nutmeg/nutmeg-plant-information.htm
http://hermionesgarden.blogspot.com/2011/02/illicium-verum-star-anise.html