Q.I’ve Stored Some Chopped Seaweed For 3 Months In Semi-sealed Black Plastic Bag. Is It Fit To Use On The Garden? It Smells Off
I wonder if anaerobic bacteria has developed. It’s slimy and has a strong smell of both the sea(as expected) and of something rotten. I don’t think a picture is needed.

Certified GKH Gardening Expert
If it smells rotten, then I wouldn't recommend using it.This could introduce some soil infections that may be very difficult to get rid of.
It is best to freeze, or compost for long term storage. Unfortunately, I do not think that it would be a good idea to use this batch.
These articles will offer some useful information: https://www.gardeningknowhow.com/composting/ingredients/composting-seaweed.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/making-fertilizer-from-seaweed.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/fertilizing-with-seaweed.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/using-kelp-fertilizer.htm