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Some of the slate steps leading to our (tiny) front porch have become seriously discolored -- a very dark gray (almost black). In the past, indoors, I used muriatic acid on slate and it did a pretty good job. However, I'm a little wary of using it again, since any run-off would likely damage our shrubs. Is there a product or a "do-it-yourself" technique that can be used to clean slate? Thank you in advance. (I have used "Wet and Forget" on some discolored shingle -- worked well. But I'm not sure the black discoloration on the slate is mould.)

Have you tried soap and water with a scrub brush?

do it in small patches use something to contain the run off. Doing it with a bucket should not big that hard to keep it away from plants

Pressure washer & clean water. No soap or detergents.

The black discoloration is probably mildew. Use the Wet & Forget.

Simple Green.

Good suggestions, people! Thank you. I'll start with the least complex and proceed from there. Does anyone know the best way to test the slate in order to determine whether the black discoloration is mildew? Thanks in advance.