It likes hot dry sunny conditions in well-drained soil. Grey downy leaves are an indicator of that. I would dig it up and put it back into the pot. Put it somewhere like a cool sunny porch. Plant out next spring (late April or May) so it has a chance to establish over a whole summer. It needs winter protection in all but the mildest areas of the uk according to the rhs website. I imagine that if you have a spot in the garden where pelargoniums will survive outdoors , the senecio would too. But like pelargoniums, it would be dodgy to plant out before winter.