How do I get rid of this this stuff on the bottom of my bathtub?

Yesterday when I went to take a shower there's was this light brown stuff on the bottom of the tub. I used some cleaner and scrubbed it off, and it came off really easily. I woke up today to find that it was there again, and there was more than yesterday! Is this stuff a fungus or something? How do I kill it and get rid of it for good?