My first grow - 2x4 tent

I was so excited to see some weed leaves pop out. Everyday the plants got noticeably bigger. I’m making sure their environment stays as consistant as possible. I did encounter my first issue. I went to refill my humidifier turns out the water had turned green. I emptied the tank into a glass you can see in the pics. Washed it and ill make sure to do so more frequently. I’m guessing it was stagnant for too long and algae grew? Has this happened to anyone? What cause it?