It doesn't look like pictures I've seen of aiptasia, but I figured I better check while they're still tiny rather than wait til they've taken over the whole tank
Its not an aiptasia (though ican see the resemblance) These two polyps have a purple encrusting foot.. starpolyps grow in this fashion so its possibly a species of these. They like good light however and are more colourful so im unsure exactly what it might be.