More info please.
Green/Blue fluffy or hairy stuff in circular blobs like on stale bread, or a white scum floating over the whole surface (like the flor that helps make Sherry), or ...
I find that I sometimes get the white stuff, especially on higher acidic fruit meads (or wines), such as Kiwi fruit, pineapple (so far).
I either scrape off with a spoon, or top-up the carboy to overflowing, and "run off" the scum. The white stuff doesn't seem overly harmful.
The Flor used in Sherry making actually seals the top from further oxidation - check the web for more info.
The Blue/green stuff I would remove asap (via a spoon). Then ensure I had the mead sulfited sufficiently (which I usually don't do at all).
If you grow your own yeast cultures, the blue/green mold means you throw the yeast culture out to avoid "bad yeast" or bacterial infection (although I have managed to remove just the portion where the blue/green mold was growing, without the rest of the culture being affected - but yeast cultures are another story
)