The concern is valid. Liquid coolant is not of particular concern when at ambient temperature. At higher temps the Phosphorous, used as a corrosion prevent agent in coolant, is a concern. It binds with the Cerium used as an Oxygen storage agent in the catalyst substrate to form CROP, Cerium Oxide Phosphorous. This makes the Cerium unavailable to passing Oxygen. This then reduces the amount of available Oxygen to react with fuel molecules. Phosphorous does not bind with the more noble metals, Platinum, Rhodium and Palladium. Note that the above is a much simplified version of the catalyst contamination processes.
It is acceptable to rinse out the catalyst with water. Reactions to not take place below 350 degrees F. It is acceptable to use a mild detergent although it is unlikely to help much unless oil based products are present. Coolant is water soluble and should rinse away with little effort. Rinsing may also remove some of the other contaminates commonly found to be clogging the pores of the substrate which can improve catalyst efficiency. Ash is usually found in catalysts and mild agitation will dislodge some of it. This is good thing.
After cleaning dry completely before installation to avoid unlikely, but possible, thermal cracking. Install with new gaskets, confirm no exhaust leaks. Run as normal, see if the on-board tests pass or fail the converter. It is my usual practice to not give up on a converter until the on-board test sets a failure trouble code.
I have cleaned failed converters with a low pressure pressure washer in an attempt to revive them. This has resulted in a few that worked for a few more months. I do not recommend this cleaning method unless one is already committed to replacement if needed.