My first recommendation is to eliminate the cause of the algea. If you have a lot of bushes and/or trees that 1) obstruct air flow, and/or 2) prevent sunlight from shining through, they should be pruned or removed.
Algea needs 2 conditions to grow: 1) little or no air flow, 2) some sunlight. Again, eliminating these conditions will solve the problem. If you were to put, say, a gritted step pad down, all you're doing is covering up the EXISTING problem and it's a matter of time before the gritted pad starts to have algea growing on it. That's how I'd slip-proof the steps.
As a general rule, I plant my flowers/bushes/trees such that when they're full grown, or grown to what I want them to grow to, they're no closer than 2-3 feet from the house wall or foundation. This allows air and sun behind the shrubbery. I'd apply the same rule to my porch and my steps and anything else. Just as an aside, this past summer I worked with one of my neighbors that had a Lilac bush growing almost 2 stories high! They should be like 3-5 feet and pruned once a year. We cut out all the 'old' growth and left the new shoots. By September, the 'bush' was about 3' high again. I told them to wait 1-2 years and they'll have a real nice bush. THEN, prune it to the desired height. This allowed air to flow around the house to the deck and it ELIMINATED the mold and mildew growing back there. (It amazed them!)