When I finished my first three-hour brew yesterday, I used a paint strainer, rather than the cotton T-shirt which is recommended in many teks, to strain the brew. I poured the filtered liquid into a clear plastic container and stored it in the fridge overnight. When I looked at the container this morning, I noticed there's quite a bit of cloudy gunk in a layer at the bottom. The upper layer is more of a clear, brownish-green color. Is the gunk at the bottom normal? Would straining with a T-shirt instead of a paint strainer have prevented it from forming?

The gunk is most likely small insoluble particles that were previously suspended in the solution and made it through your filter. They have settled to the bottom after you let it sit for a while. You can probably remove them from the brew via decanting/filtering, though it might be good to let the brew warm up to room temperature first to make sure that they aren't actives that have come out of solution in the cold temperature.

I recently was prepping for a pharma session and dissolved my harmalas + DMT fumarate in some lemonade (<5ml) but then decided to postpone it. I put my glass of liquid into the fridge. The next day, I looked in the glass and there was a large amount of crystalized alkaloids that had precipitated after the solubility was decreased by the cold temperature. Then again, I had a pretty large amount of stuff (350mg harmalas, 150mg DMT) in a tiny amount of liquid so that might have been why it happened. You're probably ok to just discard the gunk.