If the marks are on bare plastic, they should come off easily, either with a cloth dampened with water, and maybe a small amount of washing-up liquid, or with a light application of thinner, which should be allowed to evaporate, the be wipe down.
This is rather unusual for Tamiya tape, as it is relatively 'low tack'. However, as a precaution against this, I always pull a length of tape between finger and thumb, to remove some of the 'tack' before use.