To determine how much area the moss will cover after 6 months, we start with the initial area, which is 11 square centimeters, and note that this area multiplies by 1.5 each month.
After 1 month, the area will be: \[ 11 \times 1.5 = 16.5 \text{ cm}^2 \]
After 2 months, the area will be: \[ 16.5 \times 1.5 = 24.75 \text{ cm}^2 \]
After 3 months, the area will be: \[ 24.75 \times 1.5 = 37.125 \text{ cm}^2 \]
After 4 months, the area will be: \[ 37.125 \times 1.5 = 55.6875 \text{ cm}^2 \]
After 5 months, the area will be: \[ 55.6875 \times 1.5 = 83.53125 \text{ cm}^2 \]
After 6 months, the area will be: \[ 83.53125 \times 1.5 = 125.296875 \text{ cm}^2 \]
Thus, the approximate area covered by the moss when Paul returns in 6 months will be: \[ \boxed{125.3} \text{ cm}^2 \]