The tag green-construction doesn't seem to offer much specificity. It's clearly not about painting buildings green, so "green" in this sense really just means sustainable, which is already implied by the site we're on.
Thank you for posting this! The tags on this site can do with some cleaning and wiki improvements, so it's great to see people working on this.
As a moderator I could already merge the tag, but I'd like to wait a bit to see if there are any people who object to this. Meanwhile I already 'officially' proposed that the green-construction tag should be regarded as a synonym (see also https://sustainability.stackexchange.com/tags/construction/synonyms).
To quote the StackExchange explanation:
Users with a total answer score (total upvotes minus total downvotes) of 5 or more on the tag, can vote for tag synonyms. Suggestions will be automatically approved when they reach a score of 4, and automatically deleted when they reach a score of -2.