Mold issue in bathroom shower

[PJ Singh]

Hi Y'all. Please provide guidance!

Background: I have a SFH rental in Houston suburbs (cypress area) which I have rented to same tenant for past 5 years. Tenant don't want to renew lease until potential mold problem is fixed. House is a 2 story, 2002 built home and has white tile around standing shower. Main bathroom is on second floor (attic on top).

Problem: Last year during lease renewal time, they raised mold issue around main bathroom shower (closer to base). They also mentioned that water doesn't drain properly.  To re-mediate issue, I hired a handyman and re caulked the shower. I also mentioned to keep good hygiene, run exhaust fan in bathroom etc.  Now again, tenant is bringing back same mold issue. Tenant think, the mold is behind the tiles and seeps to bottom part of shower where mold grows. Last week, I got water heater replaced and during that time I mentioned plumber to check , if there is any leaks in bathroom shower. He came back with nothing, as he did not see any sign of leak ( no peeling of wall paint).

Solution: Tomorrow, I am planning to inspect myself and try to find root cause of the problem.  I will take pictures and post it here. I do think its a minor mold issue, however I do want to identify any leak problem (if any) and fix it. Here is few solutions which I am thinking:

Option 1: I was think to buy mold test kit from HD and test it myself. Also thinking of buying moisture meter to check reading. Not sure any significance to fixing problem. 

Option 2:Re grout and re caulk entire shower. Change shower fixtures (handle, head etc). May solve the problem.

Option 3:Clear the drain pipe. (Never done it, no idea how much will that cost?).

Option 4: Install motion sensor switch or exhaust and/or combine light and exhaust switch.

Option 5: Demo entire shower and try to find issue and fix it from there. ( Obviously tenant suggest this option.)

I was wondering, if some one came across this issue and how you guys fixed it. 

Note: I don't know if it matter to mention here. This mold issue and in general any other issue has been brought up frequently  in past 1-2 years ever since my original tenant moved-in officially her boyfriend. I believe, BF was living for past 3-4 years out of 5 years. Last year, I mentioned them to include BF on lease. I am feeling that he is more demanding and bring any small issues. BTW: the reason he was living with my tenant because  he was kicked out from apartment, based on rental history.

Based on my original tenant history, I do want to work with them. i believe that now they have 2 kids and want to live in nice neighborhood , they may tent to live longer here.However with her BF on board, I am also wondering should I terminate lease and move one to new tenant. Of course, there will be turnover cost ( already replaced water heater, planning to replace AC unit, flooring etc  $10K).

Any thoughts, suggestions are highly appreciated. Thank you in advance !