Orchid - Yellowing Leaf and Possible Algae
Hi all - I have a phalaenopsis for which I have 2 questions. First, recently I have noticed the growth of a green substance on some of the potting medium and pot sides. Second, the largest lower leaf of the phal has begun to turn yellow. I have posted some photos below. I would appreciate any advice from more knowledgeable folks.
Some context: The orchid seems quite young, as the total leaf span is about 6 inches. I got him from a lady at work who wasn't sure how to care for orchids. There was a tiny, dried up spike when I got him, which I cut off. I have kept the plant in a small greenhouse all winter, temp thermostatically controlled to 25 C at night and 29 C during the day. Single grow light on 14 hrs/day. RH is kept between 55% to 75%. The roots were rotting badly when i got the plant; I trimmed off the rotted parts when I repotted. Potting medium is repotme's http://www.repotme.com/orchid-potting-mix/Orchid-Mix-Phal-Dark.html and the pot is their Oxygen Core Dual Pot - 4.25". I water with filtered water with Schultz Orchid Food 19-31-17 at their recommended strength. Sometimes I add a couple drops of SuperThrive. I water whenever the top sphagnum moss is nearly dried out but not quite so - this can be as frequent as every 2 days to as little as once a week, depending on environmental factors (probably RH & dewpoint and maybe how much the orchid draws). I have a small computer fan blowing lightly in the greenhouse 24 hrs/day. I am deliberately keeping the temperature above 25 C to encourage vegetative growth and prevent the stimulation of a flower spike, as I want the orchid to increase in size and strength so that when it flowers it flowers well. I plan to drop the temperature to 20C/24C in the fall to encourage a flower spike, assuming I have gotten the plant to vegetatively grow well during the summer. A new leaf started a couple weeks ago, and I see 2 new roots forming slowly. I can't see underneath to see the condition of those roots.