Grow room walls with uncovered spray foam.

Doing a sealed room. Just foamed the walls and installed mini split. Was told I will need to paint the spray foam.. wondering what the risk of not covering it is and what the best option is for a grow room? If paint what kind ?

Not sure, but closed cell foam should technically be water proof, and not really absorb or harbor anything afaik. I could see painting it to protect from possible UV damage, or maybe for extra fireproofing? Or simply to reflect the light back. I'm not sure about how long you would want to wait for it to fully off gas or cure before coating with paint, or if that could be an issue. Personally, I would cover over it with poor mans plastex FRP panels.

Cover wall area with mylar blankets

Don't use oil based or any solvents though if you do paint. It could react or melt the foam.

I used white polyboard to cover rather than painting it

You need some surface you can clean. Foam or painted foam is not suitable.