Just because u didn't see it in the last six months doesn't mean it wasn't there.
What I'm trying to say is this, you introduced it sometimes 6 months ago.
They will come in very small, sometimes barely visible that you'll miss it. Oh those things are sneaky.
Like others have said, scrape it off gently and swiftly before it splits. It looks like its a bit malnourished based on the faded color so I doubt it spread any.
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