The LD Carlson bottles say to use a teaspoon, which is what I do and have had no ill effects from it. I get a pretty good cold break too. I would guess that too much would make your beer taste like low tide.
If you add too much you'll end up making your beer cloudier than what you started with, AND have seaweedy off flavors to deal with.
With all additions always err on the low side for safety.
A little can be good.
More is NOT better.
Before reading the directions on my LD Carlson container, I dumped all of into my 2 batch of AG. I didn't notice anything veggie like flavor wise, but I'm sure there were some off flavors that another beer snob may have picked up on.
I've wound up with a few different brands of Irish Moss at this point and they've all said 1tsp/5gal. seems to be a good rate, the beer clears pretty well after a few days in the fridge and no detectable flavor.