If the blocks were coconut coir, some can contain excess salts but I believe it depends on the brand. Some require flushing before using. I don't know whether that is still commonly the case but it used to have that reputation. I've never used it so maybe someone else can comment if indeed it was coir. Another possibility is high mineral content water but I think that's a bit of a long shot. Also, might the blocks already contain fertilizer? Perhaps you could lift off a piece of medium with the white stuff intact and take it to the terrarium shop, since they'd be able to see it in person and would presumably have experience with terrarium growing? They might even have an EC meter that could check for excess salts.
