I like to buy stepping stones made with cement and with exposed aggregates on the top. The photo below is an example of that. When I buy these used, they sometimes have 20 years of caked on mud on the top that clouds the aggregate. What is a good way to clean this off?

I use white vinegar for the most part since this is a safe acid that does a great job of softening up dirt layers. I was thinking of maybe trying citric acid, which is also a safe weak organic acid, but it is slightly stronger than citric acid.

A key requirement is that the acid needs to be safe and very tolerant to accidents. I need to be able to spill some of it on myself without creating an emergency. I need to be able to wash this off with a garden hose and have it splash on other objects and plants and not have them be put in jeopardy. It should also be environmentally safe. What are some good candidates for my application?