Yes, you could. But it won't affect the quality of the leaf mold or its ability to be used in your garden.

Inside plastic bags there will be too little air to aid the bacteria in converting those leaves and maybe too much moisture or maybe too little. There is no need to pack leaves in plastic since Ma Nature has been making leaf mold without them for eons. Plastics are also made from non renewable resources.

What he said. I used to collect bags of leaves in the fall and stack them up to use next spring when I could find bags of grass clippings to mix them with. They would come out of the bags pretty much how they went in. I've switched to putting all my leaves in a corral made of a circle of steel fencing. This way they are out in the weather so they can get air and moisture. That's how ma nature makes leaf mold anyway.

In any case, if algae grew inside the bag it would not hurt anything, it's a compost pile, stuff is supposed to grow in it. I just think you'd have a hard time controlling moisture and getting air into it.

You can have some fun with clear plastic trash bags as a solar "oven" - put in some leaves/organic matter, add a qt of water, tie up the top, and put it in the sun for a few days. Depending on what all you put in, It might be a bit whiffy when you open it, but my, it does cook things down rather quickly.

Another use is to sterilize pots and potting soil.

Advice here for people who don't have the space, or are too tidy minded, for a leaf pile is to use black plastic sacks with holes punched in. Seems a lot of effort to me but that's what's suggested.

Heard that years ago, did that years ago, and had a bag full of very wet leaves that had not been much digested in the spring. One year the leaves were so wet they froze and did not thaw out enough to use for 3 months.
I see no good reason to use non renewable resources to make something Ma Nature has been doing without them for eons.

Thank you all for your replies. i am going to find an area in the yard to create a leaf pile. I shall use chicken wire to enclose the sides - hopefully that will be enough to prevent the leaves from blowing away in the wind.

Pack them down and keep adding and compacting. After the first couple of rains and freeze-thaw cycles, they will sink down rapidly and hold together pretty well. I have made fence corrals of compacted leaves 3-4 ft. high, and by spring they sink down to a foot or so. Lots of air in leaves.