I’ve noticed that a few times in my pool, especially during warmer weeks. Even if the water looks fine and tests okay, that slick feeling on the steps usually ends up being the start of algae for me. It’s sneaky, sometimes you don’t see it, but you can feel it. I brush pretty regularly too, but it still showed up. I ended up doing a shock and tossed in a bit of algaecide, and that cleared it right up. Might be worth checking your chlorine levels later in the day too mine were fine in the morning but dropped more than I thought by the afternoon. Pools can be weird like that.