sticky substance on Phal leaves

[Miamiheat]

Hi,
3 of my phals are showing this: i find a sticky transparent substance on the edges of the leaves mostly on the bottom side of the leaf but i saw that one of them seems to produce it on the top. Plants look healthy. I cleaned one this morning just with water on some cotton and left it clean and dry. by the time i came back from work it was sticky again yet much less deposits than before. It definitely looks like the leaf is producing it.
Should I worry? Anybody with similar experience?
Thanks