Senecio mandraliscae goes dormant in the summer, which is when it's most vulnerable to overwatering. It's the nature of the plant to get leggy. Growth is from the tips of the stems, and the oldest leaves wither and fall off, leaving a denuded stem. To keep it lush and full, cut it back in spring and fall. The stem will branch where it's cut. Use the tip growth to fill gaps and discard any long, naked stems.