My guess is your pond's biology and chemistry is going through wild cycles right now due to high levels of nutrients, and you may be best served by waiting it out a little before you add another variable (alum) into the mix.

What has likely happened is you had a massive FA bloom on top of the mud particles when the wind was still and the sun was out. Then during the night the FA settled to the bottom since a lot of particles clung onto it and it stopped producing O2 bubbles. Now it is down where it is dark and will likely die, decay, and then add nutrients back into the water. Meanwhile the chemistry will jump all over the place until the cycle stabilizes and nutrients are plastered to the bottom.

Alum may wind up exposing a deeper nutrient-rich water column to the sun, making the FA go nuts making things worse.

I think with what seems to be a high level of nutrients you need to get either a bunch of pond plants established to soak them up ASAP, or pray for enough rain to flush the pond, or control through the magic of chemistry.

Note: This comes from large aquarium experience, YMMV with a pond.