Thank you for your question, Judy.  The pitch that you see is a mixture of water and sugars which were stored in the tree's roots.  A rather complex process, along with the chemical traits of water, 'pulls' this up, especially in the spring, to feed the leaves (needles) and create new growth.  The upward movement can only occur for a short period without foliage.  So, it eventually will stop, but the pitch will remain sticky (from the sugars) until they dry.  You can remove it, but setting a planter on top now will harm neither the stump nor the planter.  You may, in fact, reduce the number of sugar-loving insects, such as ants, by covering it up.  Good luck!