If your level gets covered in thin-set, sheetrock mud, mortar, etc., you gotta use acid (no, not that kind) to clean it. Get some sulfamic acid cleaner — it’s widely available at Lowe’s, Home Depot, etc. It shouldn’t melt your skin, but I always recommend wearing gloves anyway because I don’t trust long-term effects of any chemical contact.
Pour a cup or two of it into a 5-gallon bucket filled with water. Then put your encrusted level(s) in it and let it sit for 48 hours or so. The solution should do most of the cleaning itself.
For stubborn bits in crevices or corners, use a wire or nylon brush.