The geocoding field setting takes a custom field name – not an icon name. So the icon can be different for each post – just vary the value stored in that custom field for each post. For example, post ‘A’ can have a green icon and post ‘B’ can have a blue one, by placing the values ‘green-dot’ or ‘blue-dot’ in the custom field in each post.
A more flexible solution is to use the automatic icons, but in that case you must assign your posts to a taxonomy (category, tags, etc.).
If neither of those work for your site, and you know PHP, then you can code a custom routine using the filter ‘mappress_poi_iconid’. There’s an example in the documentation.