Is diluted muriatic acid harmful to grass?

1. Hydrochloric acid is one of the strongest acids extant and like any acid is most dangerous when pure or heavily concentrated. Proper precautions should be taken whenever you are dealing with acids of any kind, particularly this one (although you have obtained a large and pure enough sample to be worried is worrying itself).

2. Muriatic acid is the "kitchen name" for hydrochloric acid. It is a strong acid, and can be quite corrosive. In concentrated form it should be avoided, or used with care. If used to clean a marble floor good rubber boots and gloves are a must. In dilute form it is less of a problem. Dilute HCl (decinormal) occurs naturally in the human stomach.

You should always be careful about allowing any strong acid (whether concentrated or dilute) to come into contact with other chemicals, especially those intended to clean toilets or bleach clothes.