According to TCM theory, green tea can cause dampness (fluid/mucus retention) because it is astringent.  For example, it is used in the classic formula “chuan xiong cha tiao wan” (for wind invasion causing headache) to balance out the acrid, dispersing nature of the formula’s key medicinals, so that it doesn’t overly promote sweating (which would lead to loss of qi and fluids).  Because it is astringent, green tea can stagnate the flow of fluids in a person who suffers from dampness, and make the dampness worse.  additionally, even though green tea will also dry dampness by promoting urination, it can be overly drying and make dampness worse.  ironically, this drying quality, which stagnates the movement of fluids in the body, will create dampnes.  oolongs and pu-errhs, because they are neither hot, cold, or astringent, are best for damp/water accumulation.