My puppy had a second round of deworming 3 days ago and now has worms crawling from his bum?

I don't know what kind of worms they are, he was seen by the vet March 20th and his previous owner got a bottle of dewormer (Strongid T) and had a dose on that date. He'd pooped a few worms a few days later. He got his second dose 3 days ago and as he was peeing, I noticed a long whitish worm crawling out of his butt. Is the dewormer not working?

No thats what's supposed to happen. He is supposed to poop them out so they can leave his body

The strongid that the previous owner had given the puppy is effective on hookworms and roundworms - common for puppies. I am glad that you saw some worms come out - that was the result you want. For the long whitish worm crawling out - was it flat? If flat it might have been a tapeworm which the strongid will not kill. The tapeworm itself is actually very long and segmented and the owners usually see only the segments (size of a rice grain). You will probably want to check for your vet on this - a different medication is needed. The tapeworm is also caused by him swallowing a infected flea so make sure you have him on a flea preventative.